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