Faster joins: Fix spurious errors on incremental sync (#15232)

When pushing events in partial state rooms down incremental /sync, we
try to find the `m.room.member` state event for their senders by digging
through their auth events, so that we can present the membership to the
client. Events usually have a membership event in their auth events,
with the exception of the `m.room.create` event and a user's first join
into the room.

When implementing #13477, we took the case of a user's first join into
account, but forgot to handle the `m.room.create` case. This change
fixes that.

Signed-off-by: Sean Quah <seanq@matrix.org>