pgsql: Fix broken ruleutils support for function TRANSFORM clauses.

Fix broken ruleutils support for function TRANSFORM clauses.

I chanced to notice that this dumped core due to a faulty Assert.
To add insult to injury, the output has been misformatted since v11.
Obviously we need some regression testing here.

Discussion: https://postgr.es/m/d1cc628c-3953-4209-957b-29427acc38c8@www.fastmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d09e8789fe5ee38cfb7255871b623e211685d3a3

Modified Files
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contrib/hstore_plpython/expected/hstore_plpython.out | 16 +++++++++++++---
contrib/hstore_plpython/sql/hstore_plpython.sql      |  9 ++++++---
src/backend/utils/fmgr/funcapi.c                     |  5 +++--
3 files changed, 22 insertions(+), 8 deletions(-)