Editorial: Narrow return type of GetExportedNames.

With two assertions to make clear the validity of narrowing. This is an
exact copy of the change proposed in the PR tc39#3127(tc39#3127).

spec.html

@@ -27707,7 +27707,7 @@ <h1>
           <h1>
             GetExportedNames (
               optional _exportStarSet_: a List of Source Text Module Records,
-            ): a List of either Strings or *null*
+            ): a List of Strings
           </h1>
           <dl class="header">
             <dt>for</dt>
@@ -27723,9 +27723,11 @@ <h1>
             1. Let _exportedNames_ be a new empty List.
             1. For each ExportEntry Record _e_ of _module_.[[LocalExportEntries]], do
               1. Assert: _module_ provides the direct binding for this export.
+              1. Assert: _e_.[[ExportName]] is not *null*.
               1. Append _e_.[[ExportName]] to _exportedNames_.
             1. For each ExportEntry Record _e_ of _module_.[[IndirectExportEntries]], do
               1. Assert: _module_ imports a specific binding for this export.
+              1. Assert: _e_.[[ExportName]] is not *null*.
               1. Append _e_.[[ExportName]] to _exportedNames_.
             1. For each ExportEntry Record _e_ of _module_.[[StarExportEntries]], do
               1. Let _requestedModule_ be GetImportedModule(_module_, _e_.[[ModuleRequest]]).