Allow serialization of empty bluetooth uuids.

This change allows the passing WTF::Optional<String> types as
bluetooth.mojom.UUID optional parameter without needing to ensure the passed
object isn't empty.

BUG=None

R=juncai, dcheng

Review-Url: https://codereview.chromium.org/2646613003
Cr-Commit-Position: refs/heads/master@{#445809}
void BluetoothRemoteGATTServer::ConnectCallback(
    ScriptPromiseResolver* resolver,
    mojom::blink::WebBluetoothResult result) {
  if (!resolver->getExecutionContext() ||
      resolver->getExecutionContext()->isContextDestroyed())
    return;

  if (result == mojom::blink::WebBluetoothResult::SUCCESS) {
    setConnected(true);
    resolver->resolve(this);
  } else {
    resolver->reject(BluetoothError::take(resolver, result));
  }
}

void BluetoothRemoteGATTServer::ConnectCallback(
    ScriptPromiseResolver* resolver,
    mojom::blink::WebBluetoothResult result) {
  if (!resolver->getExecutionContext() ||
      resolver->getExecutionContext()->isContextDestroyed())
    return;

  if (result == mojom::blink::WebBluetoothResult::SUCCESS) {
    setConnected(true);
    resolver->resolve(this);
  } else {
    resolver->reject(BluetoothError::take(resolver, result));
  }
}
