Zeiss TIFF: fix used file list when expected files are missing

The top-level .tif file in particular can be missing without causing any
other problems.  This prevents any files that do not exist from
appearing on the used file list, as that would cause import into OMERO
to fail.

To test, copy one of the datasets from the ```zeiss-tiff``` directory and remove the top-level .tif file.  Without this change, ```showinf ``` on the .xml file will show the removed file in the used file list.  With this change, the removed file will not appear.