croutonwheel: Fix issue when external mouse is connected.

There are fewer valuators with external mouse compared to trackpad: we need to check if the value is empty as well (and not just `nan`).

Simply moving the mouse was causing a flood of emulated wheel clicks that would end up with Xephyr using a lot of CPU time and a sluggish system, hard to recover...

Solves @vampirexhunter & @jnack95 problem in #326, but not the original issue (that was submitted before the new `croutonwheel` anyway).