Crash in compare50 due to division by zero when includes/excludes result in zero files

compare50 crashes with a divide by zero error when the combination of --exclude and --include options results in zero files being included for a submission.

```bash
compare50 -x "*" -i "mario_1.c" -i "mario_2.c"
```
Need to handle the case where no files remain after filtering, possibly with a warning or an informative error message instead of a crash.