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)

tests/test_brew_advice.py

@@ -826,6 +826,33 @@ def test_a_pre_token_reading_is_still_guarded_against_a_stale_write(repo):
     assert after["comment"] == "corrected comment"
 
 
+def test_the_advice_is_written_for_a_reading_with_no_token_field(repo):
+    """All three shapes of "no token" must work.
+
+    A reading read from the database has no token at all; one written by
+    an older version could have the field present but empty, or null.
+    The guard matches on the absence of a token, so it has to cover all
+    three - otherwise the advice is silently never written.
+    """
+    for index, extra in enumerate(({}, {"advice_token": ""}, {"advice_token": None})):
+        reading_id = f"noToken{index:04d}"
+        repo.readings.insert_one({
+            "_id": reading_id,
+            "beer_run_id": str(index),
+            "beer_type": "Ale",
+            "sg": "1050",
+            "date": "2026-01-01",
+            "final_reading": "False",
+            "comment": "c",
+            **extra,
+        })
+
+        written = repo.set_reading_advice(reading_id, "THE ADVICE", True,
+                                           expected_sg=None,
+                                           expected_token="")
+        assert written == 1, f"the advice was not written for {extra!r}"
+
+
 def test_a_reply_cut_off_at_the_token_ceiling_is_marked(repo):
     """A reply that stopped at the token limit is incomplete.