SQL/JSON: Fix JSON_QUERY(... WITH CONDITIONAL WRAPPER)

SQL/JSON: Fix JSON_QUERY(... WITH CONDITIONAL WRAPPER)

Currently, when WITH CONDITIONAL WRAPPER is specified, array wrappers
are applied even to a single SQL/JSON item if it is a scalar JSON
value, but this behavior does not comply with the standard.

To fix, apply wrappers only when there are multiple SQL/JSON items
in the result.

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Author: Peter Eisentraut <peter@eisentraut.org>
Author: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/8022e067-818b-45d3-8fab-6e0d94d03626%40eisentraut.org
Backpatch-through: 17