Normalize the target attribute to a string

This ends up fixing a bug as sometimes "The rules for choosing a browsing context" would be invoked with name being null rather than the empty string and would therefore not end up using the "_self" code path while it should.

source

@@ -13041,7 +13041,7 @@ interface <dfn>HTMLBaseElement</dfn> : <span>HTMLElement</span> {
    <code data-x="attr-base-target">target</code> attribute of the first such <code>base</code>
    element.</p></li>
 
-   <li><p>Return null.</p></li>
+   <li><p>Return the empty string.</p></li>
   </ol>
 
   <hr>
@@ -23025,7 +23025,7 @@ document.body.appendChild(wbr);</pre>
    <li><p>Let <var>source</var> be <var>subject</var>'s <span>node document</span>'s <span
    data-x="concept-document-bc">browsing context</span>.</p></li>
 
-   <li><p>Let <var>targetAttributeValue</var> be null.</p></li>
+   <li><p>Let <var>targetAttributeValue</var> be the empty string.</p></li>
 
    <li><p>If <var>subject</var> is an <code>a</code> or <code>area</code> element, then set
    <var>targetAttributeValue</var> to the result of <span data-x="get an element's target">getting
@@ -77581,9 +77581,9 @@ console.assert(iframeWindow.frameElement === null);
   <hr>
 
   <p><dfn id="the-rules-for-choosing-a-browsing-context-given-a-browsing-context-name">The rules for
-  choosing a browsing context</dfn>, given null or a <span>browsing context name</span>
-  <var>name</var>, a <span>browsing context</span> <var>current</var>, and a
-  boolean <var>noopener</var> are as follows:</p>
+  choosing a browsing context</dfn>, given a <span>browsing context name</span> <var>name</var>, a
+  <span>browsing context</span> <var>current</var>, and a boolean <var>noopener</var> are as
+  follows:</p>
 
   <ol>
    <li><p>Let <var>chosen</var> be null.</p></li>
@@ -77658,7 +77658,7 @@ console.assert(iframeWindow.frameElement === null);
       browsing context</span> being <var>current</var>, and set <var>new</var> to true. If
       <var>name</var> is not an <span>ASCII case-insensitive</span> match for "<code
       data-x="">_blank</code>", then <var>chosen</var>'s <span data-x="browsing context
-      name">name</span> must be set to <var>name</var> (otherwise, it has no name).</p>
+      name">name</span> must be set to <var>name</var>.</p>
 
       <p class="note">If the newly created <span>browsing context</span> is immediately <span
       data-x="navigate">navigated</span>, then the navigation will be done with <span>replacement