SAM: Bugfix for 2D images

Summary: When clicked on a render window without image, eg. Sagittal, for a loaded 2D image, a corrupt image is written to disk for SAM daemon. This is diff fixes it by checking if the image is valid before writing for inferencing.

Test Plan:

1. Load a 2D image onto Workbench.
2. Init SAM tool
3. Click on a render window without the image eg. Sagittal

Expected: Inferencing is NOT carried out. Only click points exist on that render window.

Reviewers: floca, kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Maniphest Tasks: T30263

Differential Revision: https://phabricator.mitk.org/D889