fix(advice): the no-token guard must cover all three shapes of "no token"

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).

src/flask/mongo/repository.py

@@ -278,7 +278,10 @@ class BeerRepository:
             # Falling back to the gravity here would re-open the hole
             # this guard exists to close - a comment-only correction
             # leaves the gravity unchanged.
-            query["advice_token"] = {"$exists": False}
+            # `$in` with null also matches a MISSING field, so this
+            # covers a reading with no token AND one whose token field
+            # is present but empty.
+            query["advice_token"] = {"$in": [None, ""]}
         elif expected_sg is not None:
             # A direct call with a gravity to check against.
             query["sg"] = str(expected_sg)