Fix two-argument jsonb_object when called with empty arrays

Some over-eager copy-and-pasting on my part resulted in a nonsense
result being returned in this case. I have adopted the same pattern for
handling this case as is used in the one argument form of the function,
i.e. we just skip over the code that adds values to the object.

Diagnosis and patch from Michael Paquier, although not quite his
solution.

Fixes bug #13936.

Backpatch to 9.5 where jsonb_object was introduced.

src/test/regress/sql/json.sql

@@ -462,6 +462,12 @@ SELECT json_object_agg(name, type) FROM foo;
 
 -- json_object
 
+-- empty object, one dimension
+SELECT json_object('{}');
+
+-- empty object, two dimensions
+SELECT json_object('{}', '{}');
+
 -- one dimension
 SELECT json_object('{a,1,b,2,3,NULL,"d e f","a b c"}');