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}
BluetoothRemoteGATTCharacteristic* BluetoothRemoteGATTCharacteristic::create(
    ExecutionContext* context,
    mojom::blink::WebBluetoothRemoteGATTCharacteristicPtr characteristic,
    BluetoothRemoteGATTService* service,
    BluetoothDevice* device) {
  return new BluetoothRemoteGATTCharacteristic(
      context, std::move(characteristic), service, device);
}

BluetoothRemoteGATTCharacteristic* BluetoothRemoteGATTCharacteristic::create(
    ExecutionContext* context,
    mojom::blink::WebBluetoothRemoteGATTCharacteristicPtr characteristic,
    BluetoothRemoteGATTService* service,
    BluetoothDevice* device) {
  return new BluetoothRemoteGATTCharacteristic(
      context, std::move(characteristic), service, device);
}
