Bug#30897 GROUP_CONCAT returns extra comma on empty fields

The fix is a copy of Martin Friebe's suggestion.
added testing for no_appended which will be false if anything,
including the empty string is in result

mysql-test/r/func_gconcat.result

@@ -861,4 +861,13 @@ select group_concat(distinct a, c order by a desc, c desc) from t1;
 group_concat(distinct a, c order by a desc, c desc)
 31,11,10,01,00
 drop table t1;
+create table t1 (f1 char(20));
+insert into t1 values (''),('');
+select group_concat(distinct f1) from t1;
+group_concat(distinct f1)
+
+select group_concat(f1) from t1;
+group_concat(f1)
+,
+drop table t1;
 End of 5.0 tests

mysql-test/t/func_gconcat.test

@@ -590,4 +590,13 @@ select group_concat(distinct a, c order by a desc, c desc) from t1;
 drop table t1;
 
 
+#
+# Bug#30897 GROUP_CONCAT returns extra comma on empty fields
+#
+create table t1 (f1 char(20));
+insert into t1 values (''),('');
+select group_concat(distinct f1) from t1;
+select group_concat(f1) from t1;
+drop table t1;
+
 --echo End of 5.0 tests

sql/item_sum.cc

@@ -3406,7 +3406,7 @@ String* Item_func_group_concat::val_str(String* str)
   DBUG_ASSERT(fixed == 1);
   if (null_value)
     return 0;
-  if (!result.length() && tree)
+  if (no_appended && tree)
     /* Tree is used for sorting as in ORDER BY */
     tree_walk(tree, (tree_walk_action)&dump_leaf_key, (void*)this,
               left_root_right);