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`.