don't show invalid popup when kdump fields are empty

Previously, activating and then leaving a field without any
input would cause a popup saying "Invalid Entry".  Now, if the
field is empty, we won't show the error.

rhbz#867832

Change-Id: I22dd6c2d9db342b42c255c046285965c05ba1702
Signed-off-by: Mike Burns <mburns@redhat.com>