MDEV-22236 JSON_ARRAYAGG query leads to SIGSEGV in Charset::swap on optimized builds.

The json_arrayagg::val_str should handle NULL result.

mysql-test/main/func_json.result

@@ -1213,3 +1213,11 @@ DROP TABLE t1;
 #
 # End of 10.4 tests
 #
+#
+# MDEV-16620 JSON_ARRAYAGG
+#
+CREATE TABLE t1 (a INT);
+SELECT JSON_ARRAYAGG(a) FROM t1;
+JSON_ARRAYAGG(a)
+NULL
+DROP TABLE t1;

mysql-test/main/func_json.test

@@ -732,3 +732,11 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.4 tests
 --echo #
+
+-- echo #
+-- echo # MDEV-16620 JSON_ARRAYAGG
+-- echo #
+
+CREATE TABLE t1 (a INT);
+SELECT JSON_ARRAYAGG(a) FROM t1;
+DROP TABLE t1;

sql/item_jsonfunc.cc

@@ -3632,13 +3632,14 @@ String* Item_func_json_arrayagg::convert_to_json(Item *item, String *res)
 
 String* Item_func_json_arrayagg::val_str(String *str)
 {
-  str= Item_func_group_concat::val_str(str);
+  if ((str= Item_func_group_concat::val_str(str)))
+  {
     String s;
     s.append('[');
     s.swap(*str);
     str->append(s);
     str->append(']');
-
+  }
   return str;
 }