MDEV-11449 Server crashes in Item_func_or_sum::agg_item_collations.

        JSON_ARRAY didn't expect 0 arguments.

mysql-test/r/func_json.result

@@ -34,6 +34,9 @@ NULL
 select json_query('{"key1":123, "key1": [1,2,3]}', '$.key1');
 json_query('{"key1":123, "key1": [1,2,3]}', '$.key1')
 [1,2,3]
+select json_array();
+json_array()
+[]
 select json_array(1);
 json_array(1)
 [1]
@@ -150,6 +153,9 @@ json_remove(@j, '$.b')
 select json_remove(@j, '$.a');
 json_remove(@j, '$.a')
 {"b": [2, 3]}
+select json_object();
+json_object()
+{}
 select json_object("ki", 1, "mi", "ya");
 json_object("ki", 1, "mi", "ya")
 {"ki": 1, "mi": "ya"}

mysql-test/t/func_json.test

@@ -13,6 +13,7 @@ select json_query('{"key1":{"a":1, "b":[1,2]}}', '$.key1');
 select json_query('{"key1": 1}', '$.key1');
 select json_query('{"key1":123, "key1": [1,2,3]}', '$.key1');
 
+select json_array();
 select json_array(1);
 select json_array(1, "text", false, null);
 
@@ -63,6 +64,7 @@ set @j = '{"a": 1, "b": [2, 3]}';
 select json_remove(@j, '$.b');
 select json_remove(@j, '$.a');
 
+select json_object();
 select json_object("ki", 1, "mi", "ya");
 
 select json_exists('{"key1":"xxxx", "key2":[1, 2, 3]}', "$.key2");

sql/item_jsonfunc.cc

@@ -877,6 +877,14 @@ void Item_func_json_array::fix_length_and_dec()
   ulonglong char_length= 4;
   uint n_arg;
 
+  if (arg_count == 0)
+  {
+    collation.set(&my_charset_utf8_general_ci);
+    tmp_val.set_charset(&my_charset_utf8_general_ci);
+    max_length= 2;
+    return;
+  }
+
   if (agg_arg_charsets_for_string_result(collation, args, arg_count))
     return;