Faster room joins: Fix spurious error when joining a room (#13872)

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>