stbt.Keyboard.Selection: Implement `__bool__`
stbt.Keyboard.Selection: Implement `__bool__`

So that you can return `Selection(None, None)` if no selection is
detected, and callers can say `if selection:` instead of
`if selection.region:`.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester