Fix E_RECOVERABLE_ERROR in filter_draw_selection_area2()

filter_draw_selection_area2() calls current_user_get_bug_filter() which
returns false when no filter is currently defined. This value is passed
on to filter_ensure_valid_filter() which expects an array, triggering
the error.

To fix the problem we pass an empty array if there is no defined filter.

Regression introduced by 5458f7020999317b35f403fa2a0e477d6182ec12.