Fix handling of images with NaN pixel values

Summary:

• In image statistics: check if an image contains any valid pixel value at all.
• In level window: extend the search for appropriate level window values to other time steps.

Fixes T30485

Test Plan:

• Test with
  Scan_33.PA.bf.frame.0000047.nrrd1 MBDownload
• Test with other "normal" images
• Check if unit tests are still passing

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T30485

Differential Revision: https://phabricator.mitk.org/D975