Item_func has a function called is_bool_func() which returns true for
the functions that should return a boolean value, to work around the
lack of a proper BOOL datatype. Item_func_nullif is a subclass of
Item_bool_func2 and it inherits is_bool_func() from Item_func_bool which
returns true. This should not happen as NULLIF should have the datatype
of the first argument of the function.
The solution is to override is_bool_func() to return false.
A complete solution will be possible when there will be a BOOL data
type.