You can edit the project's work schedule to remove those idle days, which will suppress them from being displayed. This is a bit dangerous if those idle days might turn into non-idle days, as OP won't be scheduling any activities on those days even if it might now be otherwise possible. On the other hand, if you just need to do this on occasion to provide a document to someone else, you could just duplicate the project file, strip out the idle days, make your document, and never alter the original project file.
Send in your request for a feature to automatically hide days with no work planned with Help->Send Feedback or email
omniplan@omnigroup.com.