fix: make sure that a custom output folder for cv results is empty

[mzenk]

Issue:
When using nnUNetv2_accumulate_crossval_results and specifying an existing output folder with argument -o, the specified folder is first deleted completely and re-created before CV results are saved in it. This is unexpected behavior and might need to undesired data loss.

Fix:
Require the directory specified with -o to be non-existent or empty.

Comment on alternative solution:
Another option would be to remove the shutil.rmtree in nnunetv2/evaluation/accumulate_cv_results.py, but this would affect the case where -o is not specified in the nnUNetv2_accumulate_crossval_results entrypoint. To keep the changes minimal, the rmtree command is kept as is.

[FabianIsensee]

Good find, much appreciated!