pgsql: Handle no replica identity index case in RelationGetIdentityKeyB

Handle no replica identity index case in RelationGetIdentityKeyBitmap.

Commit e7eea52b2d has introduced a new function
RelationGetIdentityKeyBitmap which omits to handle the case where there is
no replica identity index on a relation.

Author: Mark Dilger
Reviewed-by: Takamichi Osumi, Amit Kapila
Discussion: https://www.postgresql.org/message-id/4C99A862-69C8-431F-960A-81B1151F1B89@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2731ce1bd550d08f3fdd7bcb1497af4b95170976

Modified Files
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src/backend/utils/cache/relcache.c         | 12 +++++++++++-
src/test/subscription/t/001_rep_changes.pl | 17 +++++++++++++++--
2 files changed, 26 insertions(+), 3 deletions(-)