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}
bool StructTraits<bluetooth::mojom::UUIDDataView, WTF::String>::Read(
    bluetooth::mojom::UUIDDataView data,
    WTF::String* output) {
   return data.ReadUuid(output);
 }

bool StructTraits<bluetooth::mojom::UUIDDataView, WTF::String>::Read(
    bluetooth::mojom::UUIDDataView data,
    WTF::String* output) {
   return data.ReadUuid(output);
 }
