don't print an empty line if system() produces no stderr

Change-Id: I88d85c70334de2a5f913b6c67d0d253094979fd8
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

src/shared/proparser/qmakebuiltins.cpp

@@ -265,10 +265,12 @@ void QMakeEvaluator::runProcess(QProcess *proc, const QString &command,
     proc->waitForFinished(-1);
     proc->setReadChannel(chan);
     QByteArray errout = proc->readAll();
+    if (!errout.isEmpty()) {
         if (errout.endsWith('\n'))
             errout.chop(1);
         m_handler->message(QMakeHandler::EvalError, QString::fromLocal8Bit(errout));
     }
+}
 #endif
 
 void QMakeEvaluator::populateDeps(