TwinCAT PLC/library PRs without source code changes failing

[klauer]

Issue

Intended to resolve in #108 but failed in a misguided attempt

Here's the issue:
https://github.com/pcdshub/lcls-plc-tmo-optics/actions/runs/4348701131/jobs/7597534512

The first step lists files that were changed (e.g., *.tmc or *.cmd) but the filter step (looking for TcPOU/TcGVL to check) finds nothing from that list.

The fix

The fix needs to come in the filter step - if grep fails to find anything, we revert back to the entire list of project files.