Fix thinko in JsObjectSize() macro.

Fix thinko in JsObjectSize() macro.

The macro gave the wrong answers for a JsObject with is_json == 0:
it would return 1 if jsonb_cont == NULL, or if that wasn't NULL,
it would return 1 for any non-zero size.

We could fix that, but the only use of this macro at present is in the
JsObjectIsEmpty() macro, so it seems simpler and clearer to get rid of
JsObjectSize() and put corrected logic into JsObjectIsEmpty().

Thinko in commit cf35346e8, so no need for back-patch.

Nikita Glukhov

Discussion: https://postgr.es/m/fbd1d566-bba0-a3de-d6d0-d3b1d7c24ff2@postgrespro.ru