reftable: avoid writing empty keys at the block layer

reftable: avoid writing empty keys at the block layer

The public interface (reftable_writer) already ensures that keys are
written in strictly increasing order, and an empty key by definition
fails this check.

However, by also enforcing this at the block layer, it is easier to
verify that records (which are written into blocks) never have to
consider the possibility of empty keys.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>