The problem was in that the MIN/MAX optimization in opt_sum_query was
replacing MIN/MAX functions with their constant argument without
taking into account that a query has no result rows.


mysql-test/r/func_group.result:
  Test for BUG#12882.
mysql-test/t/func_group.test:
  Test for BUG#12882.
sql/item_sum.cc:
  If it is known that a query has no result rows, do not call add()
  via the call to Item_sum::no_rows_in_result() which calls reset().
  Instead directly call clear() so that the MIN and MAX functions
  produce NULL when there are no result rows.
sql/opt_sum.cc:
  * Do not apply MIN/MAX optimization when the operand of MIN/MAX is
    a constant if it can't be determined whether the query has any
    result rows. The reason is that if the query has result rows,
    then the result of MIN/MAX is its constant argument, but if the
    query result is empty, then the result of MIN/MAX must be NULL
    irrespective of its argument.
  
  * The patch also simplifies a bit the branch that hadles COUNT().