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.