SQL/JSON: Correctly enforce the default ON EMPTY behavior

SQL/JSON: Correctly enforce the default ON EMPTY behavior

Currently, when the ON EMPTY clause is not present, the ON ERROR
clause (implicit or explicit) dictates the behavior when jsonpath
evaluation in ExecEvalJsonExprPath() results in an empty sequence.
That is an oversight in the commit 6185c9737c.

This commit fixes things so that a NULL is returned instead in that
case which is the default behavior when the ON EMPTY clause is not
present.

Reported-by: Markus Winand
Discussion: https://postgr.es/m/F7DD1442-265C-4220-A603-CB0DEB77E91D%40winand.at