Code Modernization: Correct fallback return value in get_the_author().

If the `$authordata` global is not set, `get_the_author()` returned `null`, causing a PHP 8.1 "null to non-nullable" deprecation notice in `ent2ncr()` hooked via `the_author` filter:
{{{
str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
}}}

This commit updates `get_the_author()` to return an empty string if called before `$authordata` is set, bringing consistency with a few other similar functions which also return an empty string in this case:

* `get_the_author_meta()`
* `get_the_author_posts_link()`
* `get_the_modified_author()`

Follow-up to [695/tests], [2858], [11138], [12284], [20575], [34677], [44616], [53187].

Props Soean, jrf, sabernhardt, salvoaranzulla, antpb, ebai4, sajjad67, tijmensmit, SergeyBiryukov.
Fixes #58157.

git-svn-id: https://develop.svn.wordpress.org/trunk@55755 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/author-template.php

@@ -14,11 +14,12 @@
  * Retrieves the author of the current post.
  *
  * @since 1.5.0
+ * @since 6.3.0 Returns an empty string if the author's display name is unknown.
  *
  * @global WP_User $authordata The current author's data.
  *
  * @param string $deprecated Deprecated.
- * @return string|null The author's display name.
+ * @return string The author's display name, empty string if unknown.
  */
 function get_the_author( $deprecated = '' ) {
 	global $authordata;
@@ -32,9 +33,9 @@ function get_the_author( $deprecated = '' ) {
 	 *
 	 * @since 2.9.0
 	 *
-	 * @param string|null $display_name The author's display name.
+	 * @param string $display_name The author's display name.
 	 */
-	return apply_filters( 'the_author', is_object( $authordata ) ? $authordata->display_name : null );
+	return apply_filters( 'the_author', is_object( $authordata ) ? $authordata->display_name : '' );
 }
 
 /**
@@ -55,7 +56,7 @@ function get_the_author( $deprecated = '' ) {
  *
  * @param string $deprecated      Deprecated.
  * @param bool   $deprecated_echo Deprecated. Use get_the_author(). Echo the string or return it.
- * @return string|null The author's display name, from get_the_author().
+ * @return string The author's display name, from get_the_author().
  */
 function the_author( $deprecated = '', $deprecated_echo = true ) {
 	if ( ! empty( $deprecated ) ) {
@@ -219,15 +220,15 @@ function the_author_meta( $field = '', $user_id = false ) {
 /**
  * Retrieves either author's link or author's name.
  *
- * If the author has a home page set, return an HTML link, otherwise just return the
- * author's name.
+ * If the author has a home page set, return an HTML link, otherwise just return
+ * the author's name.
  *
  * @since 3.0.0
  *
  * @global WP_User $authordata The current author's data.
  *
- * @return string|null An HTML link if the author's url exist in user meta,
- *                     else the result of get_the_author().
+ * @return string An HTML link if the author's URL exists in user meta,
+ *                otherwise the result of get_the_author().
  */
 function get_the_author_link() {
 	if ( get_the_author_meta( 'url' ) ) {
@@ -307,10 +308,11 @@ function the_author_posts() {
  *
  * @global WP_User $authordata The current author's data.
  *
- * @return string An HTML link to the author page, or an empty string if $authordata isn't defined.
+ * @return string An HTML link to the author page, or an empty string if $authordata is not set.
  */
 function get_the_author_posts_link() {
 	global $authordata;
+
 	if ( ! is_object( $authordata ) ) {
 		return '';
 	}

tests/phpunit/tests/user/getTheAuthor.php

@@ -45,4 +45,13 @@ public function test_get_the_author() {
 		$this->assertSame( $user->display_name, $author_name );
 		$this->assertSame( 'Test Author', $author_name );
 	}
+
+	/**
+	 * @ticket 58157
+	 */
+	public function test_get_the_author_should_return_empty_string_if_authordata_is_not_set() {
+		unset( $GLOBALS['authordata'] );
+
+		$this->assertSame( '', get_the_author() );
+	}
 }

tests/phpunit/tests/user/getTheAuthorMeta.php

@@ -61,7 +61,11 @@ public function test_get_the_author_meta() {
 		$this->assertSame( '', get_the_author_meta( 'does_not_exist' ) );
 	}
 
-	public function test_get_the_author_meta_no_authordata() {
+	/**
+	 * @ticket 20529
+	 * @ticket 58157
+	 */
+	public function test_get_the_author_meta_should_return_empty_string_if_authordata_is_not_set() {
 		unset( $GLOBALS['authordata'] );
 
 		$this->assertSame( '', get_the_author_meta( 'id' ) );

tests/phpunit/tests/user/getTheAuthorPostsLink.php

@@ -79,4 +79,13 @@ public function test_get_the_author_posts_link_with_permalinks() {
 
 		unset( $GLOBALS['authordata'] );
 	}
+
+	/**
+	 * @ticket 58157
+	 */
+	public function test_get_the_author_posts_link_should_return_empty_string_if_authordata_is_not_set() {
+		unset( $GLOBALS['authordata'] );
+
+		$this->assertSame( '', get_the_author_posts_link() );
+	}
 }

tests/phpunit/tests/user/getTheModifiedAuthor.php

@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * @group author
+ * @group user
+ *
+ * @covers ::get_the_modified_author
+ */
+class Tests_User_GetTheModifiedAuthor extends WP_UnitTestCase {
+	protected static $author_id = 0;
+	protected static $post_id   = 0;
+
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$author_id = $factory->user->create(
+			array(
+				'role'         => 'author',
+				'user_login'   => 'test_author',
+				'display_name' => 'Test Author',
+				'description'  => 'test_author',
+				'user_url'     => 'http://example.com',
+			)
+		);
+
+		self::$post_id = $factory->post->create(
+			array(
+				'post_author'  => self::$author_id,
+				'post_status'  => 'publish',
+				'post_content' => 'content',
+				'post_title'   => 'title',
+				'post_type'    => 'post',
+			)
+		);
+
+		add_post_meta( self::$post_id, '_edit_last', self::$author_id );
+	}
+
+	public function set_up() {
+		parent::set_up();
+
+		$GLOBALS['post'] = self::$post_id;
+	}
+
+	public function test_get_the_modified_author() {
+		$author_name = get_the_modified_author();
+		$user        = new WP_User( self::$author_id );
+
+		$this->assertSame( $user->display_name, $author_name );
+		$this->assertSame( 'Test Author', $author_name );
+	}
+
+	/**
+	 * @ticket 58157
+	 */
+	public function test_get_the_modified_author_should_return_empty_string_if_user_id_does_not_exist() {
+		update_post_meta( self::$post_id, '_edit_last', -1 );
+
+		$this->assertSame( '', get_the_modified_author() );
+	}
+}