Fix 500 error when fields missing from power_levels event
Fix 500 error when fields missing from power_levels event

If the users or events keys were missing from a power_levels event, then
we would throw 500s when trying to auth them.

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