pgsql: Fix incorrect early exit from contain_mutable_functions_walker.

Fix incorrect early exit from contain_mutable_functions_walker.

Given a JsonExpr.path_spec that is a null constant,
contain_mutable_functions_walker immediately returned false, failing
to check for other sources of mutability elsewhere in the JsonExpr.
It must only skip the jspIsMutable() test in this case.

While here, remove a couple of redundant castNode calls (I'm not
a fan of using that directly after an explicit IsA check).

Bug: #19840
Reported-by: chunling qin <303677365(at)qq(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19840-7d5243be84d7ecd3@postgresql.org
Backpatch-through: 17

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b52f3db885db1a8d8723f32db2ff35ea02ca6044

Modified Files
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src/backend/optimizer/util/clauses.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)