backends/libinput: Fix crash upon receiving motion absolute events

The input events can be processed when the workspace is not available,
e.g. during startup or shutdown, so add a corresponding guard.

As a long term plan, we need to decouple Workspace from low-level input
backend parts, but it will be a too invasive change for now.

BUG: 449317
BUG: 465456


(cherry picked from commit da26deaa)