uinput: Fix initial init

An optimization that would prevent re-opening the fd in case no keys changed
caused the init to silently "fail" due to the fact that the initial set used to
check if it could even init the fd was equal to the default (empty) set,
bypassing init entirely. This led to falling back to kwin's fake-input system,
which only works on Wayland, and crashes on non-Wayland (see #463325)


(cherry picked from commit 2578093f)