`null` index queries don't return values

[a-type]

Querying an index for the value null doesn't match anything, even if docs have null for that index.

The query is matching "null" because IDB can't query null directly, but the indexes are not writing "null". And probably this should not be "null" anyway as this may conflict with a legit string null - it should use a special value.

[a-type]

Tricker than I thought. The index in question is on a raw field, not defined separately. Manual indexes work.

This should be unblocked by recent migration refactoring which now no longer requires the whole document snapshot to be stored in the document db. I can update document db to be for (sanitized) index values only and not worry about corrupting user data with 'null' strings since the actual data is captured in operations/snapshots in the meta db.