Handle ignore_sample if no sample group is present

[johnbrvc]

Is your feature request related to a problem?
This is an enhancement for a misconfigured contest.
...

Feature Description:
If the ignore_sample grader flag is set and there is no sample test data group, just log a warning message instead of causing an error.
...

Have you considered other ways to accomplish the same thing?
The top level testdata.yaml in the data folder can be edited to remove the ignore_sample grader_flag.
...

Do you have any specific suggestions for how your feature would be implemented in PC^2?
Make a small modification to the LegacyGrader.
...

Additional context:
Noticed at the NAC2026.
...