BUG: Support parameter file with empty value

This commit fixes Slicer issue 2712

See http://na-mic.org/Mantis/view.php?id=2712
Merge branch 'slicer-issue-2712-support-parameter-file-with-empty-value'

* slicer-issue-2712-support-parameter-file-with-empty-value:
  BUG: Support parameter file with empty value