SQL/JSON: Fix issues with DEFAULT .. ON ERROR / EMPTY

SQL/JSON: Fix issues with DEFAULT .. ON ERROR / EMPTY

SQL/JSON query functions allow specifying an expression to return
when either of ON ERROR or ON EMPTY condition occurs when evaluating
the JSON path expression.  The parser (transformJsonBehavior()) checks
that the specified expression is one of the supported expressions, but
there are two issues with how the check is done that are fixed in this
commit:

* No check for some expressions related to coercion, such as
  CoerceViaIO, that may appear in the transformed user-specified
  expressions that include cast(s)

* An unsupported expression may be masked by a coercion-related
  expression, which must be flagged by checking the latter's
  argument expression recursively

Author: Jian He <jian.universality@gmail.com>
Author: Amit Langote <amitlangote09@gmail.com>
Reported-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxEqhqsfrg_p7EMyo5zak3d767iFDL8vz_4%3DZBHpOtrghw@mail.gmail.com
Discussion: https://postgr.es/m/CACJufxGOerH1QJknm1noh-Kz5FqU4p7QfeZSeVT2tN_4SLXYNg@mail.gmail.com