pgsql: If a range-partitioned table has no default partition,reject nu

If a range-partitioned table has no default partition, reject null keys.

Commit 4e5fe9ad19e14af360de7970caa8b150436c9dec introduced this
problem.  Also add a test so it doesn't get broken again.

Report by Rushabh Lathia.  Fix by Amit Langote.  Reviewed by Rushabh
Lathia and Amul Sul.  Tweaked by me.

Discussion: http://postgr.es/m/CAGPqQf0Y1iJyk4QJBdMf=pS9i6Q0JUMM_h5-qkR3OMJ-e04PyA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d7950f2222c97bd9d9f4d4edc1b59e6660c3621

Modified Files
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src/backend/catalog/partition.c      | 5 ++---
src/test/regress/expected/insert.out | 4 ++++
src/test/regress/sql/insert.sql      | 3 +++
3 files changed, 9 insertions(+), 3 deletions(-)