PERF: handle boundary case when DICOM files have empty strings as tag values

When evaluating DICOM Plugins, and empty string triggered a reload
of the file, when if fact the empty string was valid, and not
an indication of uncached tag data.

https://github.com/commontk/CTK/commit/e96b581c23c7bf9cdb10a05ef35b22aca6bbc512

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21630 3bd1e089-480b-0410-8dfb-8563597acbee