- Return empty string also if count is unsigned and value is 0


mysql-test/r/func_str.result:
  Update test result
mysql-test/t/func_str.test:
  Add test case for using an unsigned value as count parameter
  for REPEAT
sql/item_strfunc.cc:
  The repeat function should return the emptystring if
  count is unsigned and equal to zero or
  count is signed and less than or equal to zero