pgsql: Fix pg_prepared_statements.result_types for DML statements

Fix pg_prepared_statements.result_types for DML statements

Amendment to 84ad713cf85aeffee5dd39f62d49a1b9e34632da: Not all
prepared statements have a result descriptor.  As currently coded,
this would crash when reading pg_prepared_statements.  Make those
cases return null for result_types instead.  Also add a test case for
it.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6ffff0fd225432fe2ae4bd5abb7ff6113e255418

Modified Files
--------------
doc/src/sgml/catalogs.sgml            |  2 ++
src/backend/commands/prepare.c        | 20 ++++++++++++++------
src/test/regress/expected/prepare.out |  7 ++++++-
src/test/regress/sql/prepare.sql      |  4 ++++
4 files changed, 26 insertions(+), 7 deletions(-)