Fix room summary when rejected events are in state
Fix room summary when rejected events are in state

Annoyingly, `current_state_events` table can include rejected events,
in which case the membership column will be null. To work around this
lets just always filter out null membership for now.

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse