Use "child text content" to check if a <script> is empty

This matches 2/4 implementations, and is simpler and integrates better
with the rest of the algorithm.

Closes #3419.

source

@@ -57723,11 +57723,12 @@ o............A....e
 
    </li>
 
+   <li><p>Let <var>source text</var> be the element's <span>child text content</span>.</p></li>
+
    <li id="script-processing-empty">
 
-    <p>If the element has no <code data-x="attr-script-src">src</code> attribute, and its child
-    nodes, if any, consist only of comment nodes and empty <code>Text</code> nodes, then return. The
-    script is not executed.</p>
+    <p>If the element has no <code data-x="attr-script-src">src</code> attribute, and <var>source
+    text</var> is the empty string, then return. The script is not executed.</p>
 
    </li>
 
@@ -57845,8 +57846,8 @@ o............A....e
    data-x="attr-script-src">src</code> content attribute, and the <span>Should element's inline
    behavior be blocked by Content Security Policy?</span> algorithm returns "<code
    data-x="">Blocked</code>" when executed upon the <code>script</code> element, "<code
-   data-x="">script</code>", and the <code>script</code> element's <span>child text content</span>,
-   then return. The script is not executed. <ref spec="CSP"></p></li>
+   data-x="">script</code>", and <var>source text</var>, then return. The script is not executed.
+   <ref spec="CSP"></p></li>
 
    <li id="script-processing-for">
 
@@ -58006,9 +58007,6 @@ o............A....e
     run these substeps:</p>
 
     <ol>
-     <li><p>Let <var>source text</var> be the <code>script</code> element's <span>child text
-     content</span>.</p></li>
-
      <li><p>Let <var>base URL</var> be the <code>script</code> element's <span>node
      document</span>'s <span>document base URL</span>.</p></li>