The change_detector script raised ValueError on scheduled workflow
runs, breaking the daily CodeQL scan. Add a schedule branch alongside
the existing workflow_dispatch branch since both trigger types aren't
tied to a specific diff.
While here, fix a latent bug: the workflow_dispatch branch printed
"assuming all changed" but actually triggered nothing - it left
files = [], and detect_changes([], ...) returns False, so no group
flag was set in the output. Setting files = None makes the
"files is None or detect_changes(...)" short-circuit fire for every
group, matching the printed intent. Same fix benefits both event
types.
Other touch-ups:
- Add the unsupported event name to the error message so future
surprises are easier to debug.
- Type files as Optional[List[str]] (mypy required this once the
None branch was added).
- Guard the len(files) check against None.
