API: `is_screen_black`: Make `frame` parameter optional

This makes it consistent with the other functions such as `match()`, and
makes it more useful with a hypothetical `wait_until`, allowing
constructions such as:

    assert wait_until(is_screen_black)
Merge branch 'is_screen_black-optional-frame' into one

Conflicts:
	tests/test-stbt-py.sh