When a room has the partial state flag, we may not have an accurate
`m.room.member` event for event senders in the room's current state, and
so cannot perform soft fail checks correctly. Skip the soft fail check
entirely in this case.
As an alternative, we could block until we have full state, but that
would prevent us from receiving incoming events over federation, which
is undesirable.
Signed-off-by: Sean Quah <seanq@matrix.org>
