Fix missing null character check on guest_access room state

When updating room_stats_state, we try to check for null bytes slipping
in to the
content for state events. It turns out we had added guest_access as a
field to
room_stats_state without including it in the null byte check.

Lo and behold, a null byte in a m.room.guest_access event then breaks
room_stats_state
updates.

This PR adds the check for guest_access. A further PR will improve this
function so that this hopefully does not happen again in future.

synapse/storage/databases/main/stats.py

@@ -210,6 +210,7 @@ class StatsStore(StateDeltasStore):
         * topic
         * avatar
         * canonical_alias
+        * guest_access
 
         A is_federatable key can also be included with a boolean value.
 
@@ -234,6 +235,7 @@ class StatsStore(StateDeltasStore):
             "topic",
             "avatar",
             "canonical_alias",
+            "guest_access",
         ):
             field = fields.get(col, sentinel)
             if field is not sentinel and (not isinstance(field, str) or "\0" in field):