pgsql: Fix plpgsql's handling of "return simple_record_variable".

Fix plpgsql's handling of "return simple_record_variable".

If the variable's value is null, exec_stmt_return() missed filling
in estate->rettype. This is a pretty old bug, but we'd managed not
to notice because that value isn't consulted for a null result ...
unless we have to cast it to a domain. That case led to a failure
with "cache lookup failed for type 0".

The correct way to assign the data type is known by exec_eval_datum.
While we could copy-and-paste that logic, it seems like a better
idea to just invoke exec_eval_datum, as the ROW case already does.

Reported-by: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAFj8pRBT_ahexDf-zT-cyH8bMR_qcySKM8D5nv5MvTWPiatYGA@mail.gmail.com
Backpatch-through: 14

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ce4b7e3a1058eb3edd7511f2770379d7f78bd3a0

Modified Files
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src/pl/plpgsql/src/expected/plpgsql_domain.out | 13 +++++++++++++
src/pl/plpgsql/src/pl_exec.c | 20 +++-----------------
src/pl/plpgsql/src/sql/plpgsql_domain.sql | 10 ++++++++++
3 files changed, 26 insertions(+), 17 deletions(-)