During a `lazy_load_members` `/sync`, we look through auth events in
rooms with partial state to find prior membership events. When such a
membership is not found, an error is logged.
Since the first join event for a user never has a prior membership event
to cite, the error would always be logged when one appeared in the room
timeline.
Avoid logging errors for such events.
Introduced in #13477.
Signed-off-by: Sean Quah <seanq@matrix.org>
