pgsql: Fix CREATE VIEW to allow zero-column views.

Fix CREATE VIEW to allow zero-column views.

We should logically have allowed this case when we allowed zero-column
tables, but it was overlooked.

Although this might be thought a feature addition, it's really a bug
fix, because it was possible to create a zero-column view via
the convert-table-to-view code path, and then you'd have a situation
where dump/reload would fail. Hence, back-patch to all supported
branches.

Arrange the added test cases to provide coverage of the related
pg_dump code paths (since these views will be dumped and reloaded
during the pg_upgrade regression test). I also made them test
the case where pg_dump has to postpone the view rule into post-data,
which disturbingly had no regression coverage before.

Report and patch by Ashutosh Sharma (test case by me)

Discussion: https://postgr.es/m/CAE9k0PkmHdeSaeZt2ujnb_cKucmK3sDDceDzw7+d5UZoNJPYOg@mail.gmail.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0b476cf623dd5849679d4509f6b06b8be7f3436e

Modified Files
--------------
src/backend/commands/view.c | 5 -----
src/test/regress/expected/create_view.out | 10 ++++++++++
src/test/regress/expected/rules.out | 8 ++++++++
src/test/regress/expected/sanity_check.out | 1 +
src/test/regress/sql/create_view.sql | 12 ++++++++++++
5 files changed, 31 insertions(+), 5 deletions(-)