When using a file parameter, and no file is selected in the build trigger view,
hudson should not 'touch' (create a zero-length) file at the location specified
in the parameter definition.

My use case is to use a file parameter to temporarily ovewrite a project file
that controls the build process. If I don't specify any file in the build page,
I don't want hudson to overwrite my default control file (coming from svn) with
a zero length one.