func_json: Fix JSON parsing issues.

Fix issue with returning empty instead of dumping
the JSON string when recursing.

Also adds a unit test to capture this fix.

ASTERISK-30441 #close

Change-Id: If0bde9f3fe84f7af485e0838205cc21e0f752a85
(cherry picked from commit f9fd76677f20f9db05c26ac55cc064394feb8650)