MDEV-11445 JSON_MERGE requires at least two arguments in MySQL, but not in MariaDB.

        Create_func_json_merge::create_native fixed.

mysql-test/r/func_json.result

@@ -197,6 +197,8 @@ t1	CREATE TABLE `t1` (
   `json_quote('foo')` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+select json_merge('string');
+ERROR 42000: Incorrect parameter count in the call to native function 'json_merge'
 select json_merge('string', 123);
 json_merge('string', 123)
 ["string", 123]

mysql-test/t/func_json.test

@@ -83,6 +83,8 @@ select * from t1;
 show create table t1;
 drop table t1;
 
+--error ER_WRONG_PARAMCOUNT_TO_NATIVE_FCT
+select json_merge('string');
 select json_merge('string', 123);
 
 select json_type('{"k1":123, "k2":345}');

sql/item_create.cc

@@ -5192,7 +5192,7 @@ Create_func_json_merge::create_native(THD *thd, LEX_STRING name,
   int arg_count;
 
   if (item_list == NULL ||
-      (arg_count= item_list->elements) == 0)
+      (arg_count= item_list->elements) < 2) // json, json
   {
     my_error(ER_WRONG_PARAMCOUNT_TO_NATIVE_FCT, MYF(0), name.str);
     func= NULL;