[Clipboard APIs] Handle null values for blobs OnRead()

Don't create |V8UnionBlobOrString| if the blob is null so that the null
value is handled in |ClipboardPromise::ResolveRead()| and it doesn't
perform an invalid conversion |ToResolvedPromise|.

Updated |ClipboardItemWithDOMStringSupport| flag status to experimental

Bug: 377251898
Change-Id: I47400f8a7671e4bb7b98b44d864b24b510e64835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5993934
Reviewed-by: Christine Hollingsworth <[email protected]>
Reviewed-by: Nate Chapin <[email protected]>
Reviewed-by: Ayu Ishii <[email protected]>
Commit-Queue: Sambamurthy Bandaru <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1379178}

clipboard-apis/async-write-blobs-read-blobs.https.html

@@ -45,4 +45,18 @@
   assert_equals(blobTextOutput.type, 'text/plain');
   assert_equals(blobImageOutput.type, 'image/png');
 }, 'Verify write and read clipboard (multiple types)');
+
+promise_test(async () => {
+  await tryGrantReadPermission();
+  await tryGrantWritePermission();
+  const text_blob = new Blob(['hello'], {type: 'text/plain'});
+  const html_blob = new Blob(undefined, {type: 'text/html'});
+  const item = new ClipboardItem({'text/plain': text_blob, 'text/html': html_blob});
+
+  await waitForUserActivation();
+  await navigator.clipboard.write([item]);
+  await waitForUserActivation();
+  const [clipboardItem] = await navigator.clipboard.read();
+}, 'navigator.clipboard.read() does not crash if clipboard data has null values');
+
 </script>