MDEV-11446 JSON_MERGE accepts arguments in invalid format.

        Test case added.

mysql-test/r/func_json.result

@@ -234,6 +234,9 @@ json_merge('{"1": 2}', '{"true": false}', '{"3": 4}')
 select json_merge(NULL,json_object('foo', 1));
 json_merge(NULL,json_object('foo', 1))
 NULL
+select json_merge('a','b');
+json_merge('a','b')
+NULL
 select json_type('{"k1":123, "k2":345}');
 json_type('{"k1":123, "k2":345}')
 OBJECT

mysql-test/t/func_json.test

@@ -98,6 +98,7 @@ select json_merge('[1, 2]', '[true, false]');
 select json_merge('{"1": 2}', '{"true": false}');
 select json_merge('{"1": 2}', '{"true": false}', '{"3": 4}');
 select json_merge(NULL,json_object('foo', 1));
+select json_merge('a','b');
 
 select json_type('{"k1":123, "k2":345}');
 select json_type('[123, "k2", 345]');