lexgen empty union (#1115)
This fixes lexgen for empty unions (which it now treats similarly to
`unknown`).
Also handles the special "meta" unknown field in
`tools.ozone.moderation.defs#modTool`, which is a "true" unknown (aka,
it will not have `$type`, similar to plc ops and did docs).
I tested running against atproto main, and the API part worked, but
current lexgen adds yet more params to queryEvents which breaks things.
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