pgsql: Fix check_exclusion_or_unique_constraint for UNIQUE NULLS NOT DI

Fix check_exclusion_or_unique_constraint for UNIQUE NULLS NOT DISTINCT.

Adjusting this function was overlooked in commit 94aa7cc5f.  The only
visible symptom (so far) is that INSERT ... ON CONFLICT could go into
an endless loop when inserting a null that has a conflict.

Richard Guo and Tom Lane, per bug #17558 from Andrew Kesper

Discussion: https://postgr.es/m/17558-3f6599ffcf52fd4a@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3419d51e1957855f527b94ae983fe8503aafe15c

Modified Files
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src/backend/executor/execIndexing.c       | 18 ++++++++++++------
src/test/regress/expected/constraints.out |  5 +++--
src/test/regress/sql/constraints.sql      |  5 +++--
3 files changed, 18 insertions(+), 10 deletions(-)