MDEV-11447 JSON_MERGE merges valid JSON objects incorrectly.

        Test case added.

mysql-test/r/func_json.result

@@ -237,6 +237,9 @@ NULL
 select json_merge('a','b');
 json_merge('a','b')
 NULL
+select json_merge('{"a":"b"}','{"c":"d"}');
+json_merge('{"a":"b"}','{"c":"d"}')
+{"a":"b", "c":"d"}
 select json_type('{"k1":123, "k2":345}');
 json_type('{"k1":123, "k2":345}')
 OBJECT

mysql-test/t/func_json.test

@@ -99,6 +99,7 @@ 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_merge('{"a":"b"}','{"c":"d"}');
 
 select json_type('{"k1":123, "k2":345}');
 select json_type('[123, "k2", 345]');