Let a controller claim the empty Player 1 slot on first input (#1864)

When slot 0 was assigned to a controller that is no longer connected,
noteGamepadButton bailed out because there was no occupant to displace,
so a built-in controller stuck on P2 could never regain P1. Treat an
empty or disconnected P1 slot as claimable: the first device that sends
real gamepad input takes slot 0. Connected active occupants are still
never displaced.

Co-authored-by: Utkarsh Dalal <utkarsh.dalal@toptal.com>