osk-sdl crashes if /dev/input doesn't exist

As discussed in #osk-sdl-dev:postmarketos.org, when running pmOS with FDE in Qemu, osk-sdl crashes for me because /dev/input doesn't exist.

# DFBARGS=system=fbdev,no-cursor,disable-module=ps2mouse SDL_VIDEODRIVER=directfb OSK_EXTRA_ARGS=--no-gles osk-sdl $OSK_EXTRA_ARGS -n root -d /dev/vda2 -c /etc/osk.conf -o /boot/osk.conf -v
INFO: osk-sdl v0.66
ERROR: Could not open config file: /boot/osk.conf
ERROR: Config override file could not be loaded, continuing
terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'
  what():  filesystem error: directory iterator cannot open directory: No such file or directory [/dev/input]
Aborted

Since the directory is only used to check for a physical keyboard, which is optional, its absence should be handled gracefully.