`redbiom search metadata` should support finding samples where a category is not null

[wasade]

It would be pleasant to be able to do:

$ redbiom search metadata "where foo is not None"

And the result would be any sample where that category foo exists

[wasade]

Instead of using the query structures, it may be better to capture this and query directly. We should be storing what samples have what metadata category already in redis so I think it's just an SMEMBERS call