Fix overly-lax regex pattern in TAP test of READ_REPLICATION_SLOT

Fix overly-lax regex pattern in TAP test of READ_REPLICATION_SLOT

The case checking for a NULL output when a slot does not exist was
too lax, as it was passing for any output generated by the query.  This
fixes the matching pattern to be what it should be, matching only on
"||".

Oversight in b4ada4e.