Problem: setting Item_func_rollup_const::null_value property to argument's null_value
before (without) the argument evaluation may result in a crash due to wrong null_value.

Fix: use is_null() to set Item_func_rollup_const::null_value instead as it evaluates
the argument if necessary and returns a proper value.


mysql-test/r/olap.result:
  Fix for bug #32558: group by null-returning expression with rollup causes crash
    - test result.
mysql-test/t/olap.test:
  Fix for bug #32558: group by null-returning expression with rollup causes crash
    - test case.
sql/item_func.h:
  Fix for bug #32558: group by null-returning expression with rollup causes crash
    - use args[0]->is_null() to obtain Item_func_rollup_const::null_value 
      instead of args[0]->null_value as it's not set in advance in case of 
      constant functions.