Clipboard API: Add nullptr check to avoid DCHECK renderer crash.

Return early if utf_text.data() == nullptr. utf_text.data() can be
nullptr if the Clipboard was empty. Previously, this would trigger a
DCHECK in Blob::Create().

Tested manually using `pbcopy < /dev/null`, as per bug repro steps.
Unfortunately, Chrome's test infra doesn't seem to have a way to
simulate clearing the clipboard. Therefore, there isn't an automated
test being added here to verify this behavior.

Bug: 1071279
Change-Id: I95cdc6ed2df25394ca1ffa78d4cde96782d9a10f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151132
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759794}

third_party/blink/renderer/modules/clipboard/clipboard_reader.cc

@@ -55,6 +55,9 @@ class ClipboardTextReader final : public ClipboardReader {
   Blob* ReadFromSystem() override {
     String plain_text =
         system_clipboard()->ReadPlainText(mojom::ClipboardBuffer::kStandard);
+    // |plain_text| is empty if the clipboard is empty.
+    if (plain_text.IsEmpty())
+      return nullptr;
 
     // Encode WTF String to UTF-8, the standard text format for blobs.
     StringUTF8Adaptor utf_text(plain_text);

third_party/blink/renderer/modules/clipboard/clipboard_reader.h

@@ -25,6 +25,7 @@ class ClipboardReader : public GarbageCollected<ClipboardReader> {
                                  const String& mime_type);
   virtual ~ClipboardReader();
 
+  // Returns nullptr if the data is empty or invalid.
   virtual Blob* ReadFromSystem() = 0;
 
   void Trace(Visitor* visitor);