pgsql: Fix not-null constraint test

Fix not-null constraint test

When a partitioned table has a primary key, trying to find the
corresponding not-null constraint for that column would come up empty,
causing code that's trying to check said not-null constraint to crash.
Fix by only running the check when the not-null constraint exists.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/d57b4a69-7394-3146-5976-9a1ef27e7972@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0ec2ddbe088f6da35444fad688a62eae4fbd840

Modified Files
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src/backend/commands/tablecmds.c          | 26 +++++++++++++++++++-------
src/test/regress/expected/constraints.out | 11 +++++++++++
src/test/regress/sql/constraints.sql      | 11 +++++++++++
3 files changed, 41 insertions(+), 7 deletions(-)