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:
  test result
mysql-test/t/func_gconcat.test:
  test case
sql/item_sum.cc:
  added testing for no_appended which will be False if anything,
  including the empty string is in result