[IZPACK-1085]

A fix for http://jira.codehaus.org/browse/IZPACK-1085 (In Console mode file field cannot be empty despite "allowEmptyValue=true")

Explanation: Java's java.io.File will substitute an empty path for some system-dependent path (in this case the path from where the installer is run), so the FileFieldView's validate() method wasn't receiving a blank path at all and so the "allowEmptyValue" attribute was irrelevant.