Type_handler_xxx::Item_const_eq() can handle only non-NULL values.
The code in Item_basic_value::eq() did not take this into account.

Adding a test to detect three different combinations:
- Both values are NULLs, return true.
- Only one value is NULL, return false.
- Both values are not NULL, call Type_handler::Item_const_eq()
  to check equality.