The problem was that Item_sum_hybrid::val_xxx() did not propagate null values
up the expression tree.


mysql-test/r/func_time.result:
  New test case.
mysql-test/t/func_time.test:
  New test case.
sql/item_sum.cc:
  Check for null_value when evaluating sub-items in sub-trees in Item_sum_hybrid::val_xxx()