Handle TimelineBatch being limited and empty.
Handle TimelineBatch being limited and empty.

This hopefully addresses #5407 by gracefully handling an empty but
limited TimelineBatch. We also add some logging to figure out how this
is happening.

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