rootston: fix startup with no config
Would choke on NULL name on strcmp later on: #0 0x00007ffff6e0ad4a in __strcmp_sse2_unaligned () at /usr/lib/libc.so.6 #1 0x00005555555593d2 in config_get_keyboard (config=0x555555774f80, device=0x555555775768) at ../rootston/config.c:443 #2 0x000055555555d7e3 in keyboard_add (device=0x555555775768, input=0x555555e96d70) at ../rootston/keyboard.c:206 #3 0x000055555555ca39 in input_add_notify (listener=0x555555e96fa8, data=0x555555775768) at ../rootston/input.c:34 #4 0x00007ffff7ba2b80 in wl_signal_emit (signal=0x5555557754f8, data=0x555555775768) at /usr/include/wayland-server-core.h:388 #5 0x00007ffff7ba37e0 in wlr_x11_backend_start (backend=0x5555557754f0) at ../backend/x11/backend.c:286 #6 0x00007ffff7b9372d in wlr_backend_start (backend=0x5555557754f0) at ../backend/backend.c:29 #7 0x000055555555dcc0 in main (argc=1, argv=0x7fffffffe408) at ../rootston/main.c:39