Fix NULL input behaviour of pg_stat_get_replication_slot().
pg_stat_get_replication_slot() accidentally was marked as non-strict, crashing
when called with NULL input. As it's already released, introduce an explicit
NULL check in 14, fix the catalog in HEAD.
Bumps catversion in HEAD.
Discussion: https://postgr.es/m/
20220326212432.s5n2maw6kugnpyxw@alap3.anarazel.de
Backpatch: 14-, where replication slot stats were introduced