Ensure that DateTimeFormatter doesn't format an empty string

This fixes zendframework/zendframework#4393 and maintains BC.

src/DateTimeFormatter.php

@@ -71,7 +71,9 @@ public function filter($value)
      */
     protected function normalizeDateTime($value)
     {
-        if (is_int($value)) {
+        if (empty($value)) {
+            return $value;
+        } elseif (is_int($value)) {
             $dateTime = new DateTime('@' . $value);
         } elseif (!$value instanceof DateTime) {
             $dateTime = new DateTime($value);

test/DateTimeFormatterTest.php

@@ -33,6 +33,15 @@ public function tearDown()
         date_default_timezone_set($this->defaultTimezone);
     }
 
+    public function testFormatterDoesNotFormatAnEmptyString()
+    {
+        date_default_timezone_set('UTC');
+
+        $filter = new DateTimeFormatter();
+        $result = $filter->filter('');
+        $this->assertEquals('', $result);
+    }
+
     public function testDateTimeFormatted()
     {
         date_default_timezone_set('UTC');