[dart:js_interop] Specify that null or empty @js values are ignored

CoreLibraryReviewExempt: Doc-only change.
Change-Id: I75b729f638b6fff9ee8969c8ff90511ee44f147d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443741
Auto-Submit: Srujan Gaddam <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>
Reviewed-by: Erik Ernst <[email protected]>

Author: srujzs

sdk/lib/js_interop/js_interop.dart

@@ -62,8 +62,8 @@ export 'dart:js_util' show NullRejectionException;
 /// - Renaming external declarations by parameterizing the annotation on the
 ///   member with [name].
 ///
-/// In the case where [name] is not specified, the Dart name of the extension
-/// type or external declaration is used as the default.
+/// In the case where [name] is not specified, is `null`, or is empty, the Dart
+/// name of the extension type or external declaration is used as the default.
 ///
 /// See https://dart.dev/interop/js-interop/usage#js for more details on how to
 /// use this annotation.