Hide internal error for pg_collation_actual_version(<bad OID>).

 Hide internal error for pg_collation_actual_version(<bad OID>).
 
 Instead of an unsightly internal "cache lookup failed" message, just
 return NULL for bad OIDs, as is the convention for other similar things.
 
 Reported-by: Justin Pryzby <pryzby@telsasoft.com>
 Reviewed-by: Michael Paquier <michael@paquier.xyz>
 Discussion: https://postgr.es/m/20210117215940.GE8560%40telsasoft.com