Allow null to be passed as output method in zwlr_screencopy_manager_v1::capture_output and zwlr_screencopy_manager_v1::capture_output_region

Allowing a server to decide which output should be saved in cases were null is passed in zwlr_screencopy_manager_v1::capture_output, or zwlr_screencopy_manager_v1::capture_output_region will drastically simplify a way to screenshot one specific screen that the user might want. Which feels reasonable as a similar example were you might want to create a pop-up on the screen that server will choose is already handled the same way in zwlr_layer_shell_v1::get_layer_surface

In addition it seems like this functionality might be the only reason why issue #470 even exists, so this change will probably result in #470 becoming obsolete.