Make script's "base URL" member nullable

Various places in the specification expected it to sometimes be null, but the type signature did not declare it as nullable. Fix the type signature.

source

@@ -97857,10 +97857,10 @@ document.querySelector("button").addEventListener("click", bound);
    <dt>A <dfn for="script" export data-x="concept-script-base-url">base
    URL</dfn></dt>
 
-   <dd><p>A base <span>URL</span> used for <span data-x="resolve a module specifier">resolving
-   module specifiers</span>. This will either be the URL from which the script was obtained, for
-   external scripts, or the <span>document base URL</span> of the containing document, for inline
-   scripts.</p></dd>
+   <dd><p>Null or a base <span>URL</span> used for <span data-x="resolve a module
+   specifier">resolving module specifiers</span>. When non-null, this will either be the URL from
+   which the script was obtained, for external scripts, or the <span>document base URL</span> of the
+   containing document, for inline scripts.</p></dd>
   </dl>