Add 'allow empty' support to views_handler_filter_in_operator

As discussed in CCK's #311912: Many_to_one filters/args handlers, attached patch adds optional 'is empty / is not empty' operators to views_handler_filter_in_operator handler.
We originally considered adding this to views_handler_filter_many_to_one (I have working code for this as well), but adding it to the parent class is actually more straightforward.

The code is largely inspired by views_handler_filter_string / views_handler_filter_numeric.