Use optional for ReelAsset _annotation_text.

Use optional for ReelAsset _annotation_text.

Not only is this tag optional in Interop and SMPTE, but it would
appear that if it is present but empty a DCP will not play back
on Sony SRX320 / LMT3000 systems (DoM bug #2124).

Here we use optional<>, as seems to make sense, and also refuse
to write empty tags (instead omitting the tag entirely).