BugFix: nnUnet can be now used for mono modal as well. No.of extra modalities won't be checked for mono modal Previews anymore.

Summary:
The if-case check is corrected removing the unnecessary check for no.of modalities for mono modal data Preview.
Ref Task: T29059

Test Plan: NA

Reviewers: kislinsk, kalali, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Differential Revision: https://phabricator.mitk.org/D615