Problem: lying to the optimizer that a function (Item_func_inet_ntoa)
cannot return NULL values leads to unexpected results (in the case group
keys creation/comparison is broken). 

Fix: Item_func_inet_ntoa::maybe_null should be set properly.


mysql-test/r/func_misc.result:
  Fix for bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'
    - test result.
mysql-test/t/func_misc.test:
  Fix for bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'
    - test case.
sql/item_strfunc.h:
  Fix for bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'
    - set Item_func_inet_ntoa::maybe_null flag.