The guard matches on the absence of a token. `{"$exists": False}` covers
a reading with no token field, but NOT one whose token field is present
and empty.
Verified against the live database, before the fix:
token present but empty -> write matched: 0 <-- missed it
`{"$in": [None, ""]}` covers all three - a missing field, an empty
field, and a null one - because `$in` with null also matches a missing
field.
no token field -> write matched: 1
token present but empty -> write matched: 1
token is null -> write matched: 1
183 tests (up from 182).
hermes
4bedd23c24