Fix missing null character check on guest_access room state (#8373)

When updating the `room_stats_state` table, 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`.

changelog.d/8373.bugfix

@@ -0,0 +1 @@
+Include `guest_access` in the fields that are checked for null bytes when updating `room_stats_state`. Broke in v1.7.2.

synapse/storage/databases/main/stats.py

@@ -210,6 +210,7 @@ async def update_room_state(self, room_id: str, fields: Dict[str, Any]) -> None:
         * topic
         * avatar
         * canonical_alias
+        * guest_access
 
         A is_federatable key can also be included with a boolean value.
 
@@ -234,6 +235,7 @@ async def update_room_state(self, room_id: str, fields: Dict[str, Any]) -> None:
             "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):