Desktop: clear notes on empty dive

If no dive is set, all fields except the note field were cleared.
Also clear notes.

Fixes #2172

Reported-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/tab-widgets/maintab.cpp

@@ -589,6 +589,7 @@ void MainTab::updateDiveInfo()
 		ui.buddy->clear();
 		ui.airtemp->clear();
 		ui.watertemp->clear();
+		ui.notes->clear();
 		/* set date and time to minimums which triggers showing the special value text */
 		ui.dateEdit->setSpecialValueText(QString("-"));
 		ui.dateEdit->setMinimumDate(QDate(1, 1, 1));