config: fix NULL value detection for ID_INPUT being unset

Erroneous condition caused us to keep going with all devices that didn't have
ID_INPUT set.

Fixes: 5aad81445c
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104382
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

config/udev.c

@@ -135,7 +135,7 @@ device_added(struct udev_device *udev_device)
 #endif
 
     value = udev_device_get_property_value(udev_device, "ID_INPUT");
-    if (value && !strcmp(value, "0")) {
+    if (!value || !strcmp(value, "0")) {
         LogMessageVerb(X_INFO, 10,
                        "config/udev: ignoring device %s without "
                        "property ID_INPUT set\n", path);