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}
DEFINE_TRACE(BluetoothRemoteGATTServer) {
  visitor->trace(m_activeAlgorithms);
  visitor->trace(m_device);
}

DEFINE_TRACE(BluetoothRemoteGATTServer) {
  visitor->trace(m_activeAlgorithms);
  visitor->trace(m_device);
}
