[libcalamaresui] Update progress message only if it is non-empty

This improves the situation for jobs that do not provide
a status: their blank status does not overwrite the status
bar, and since (previous commit) the description or name
is used to start the job if the status is empty, at least
**something** is displayed.

SEE #1528

src/libcalamaresui/viewpages/ExecutionViewStep.cpp

@@ -191,8 +191,11 @@ void
 ExecutionViewStep::updateFromJobQueue( qreal percent, const QString& message )
 {
     m_progressBar->setValue( int( percent * m_progressBar->maximum() ) );
+    if ( !message.isEmpty() )
+    {
         m_label->setText( message );
     }
+}
 
 void
 ExecutionViewStep::onLeave()