fix JSON_ARRAYAGG not to over-quote json in joins

use metadata (in particular is_json() property) of the original
argument item, even if the actual argument was later replaced
with an Item_temptable_field

mysql-test/main/func_json.result

@@ -1376,9 +1376,9 @@ SELECT t1.id, JSON_ARRAYAGG(JSON_OBJECT('id',t2.id)) as materials
 from t1 LEFT JOIN t2 on t1.id = t2.owner_id
 GROUP BY t1.id ORDER BY id;
 id	materials
-1	["{\"id\": 1}","{\"id\": 2}"]
-2	["{\"id\": 3}"]
-3	["{\"id\": 4}"]
+1	[{"id": 1},{"id": 2}]
+2	[{"id": 3}]
+3	[{"id": 4}]
 DROP TABLE t1;
 DROP TABLE t2;
 #

sql/item_jsonfunc.cc

@@ -3723,12 +3723,12 @@ String *Item_func_json_arrayagg::get_str_from_item(Item *i, String *tmp)
 }
 
 
-String *Item_func_json_arrayagg::get_str_from_field(Item *i,Field *f,
+String *Item_func_json_arrayagg::get_str_from_field(Item *, Field *f,
     String *tmp, const uchar *key, size_t offset)
 {
   m_tmp_json.length(0);
 
-  if (append_json_value_from_field(&m_tmp_json, i, f, key, offset, tmp))
+  if (append_json_value_from_field(&m_tmp_json, *orig_args, f, key, offset, tmp))
     return NULL;
 
   return &m_tmp_json;