Only emit empty() in SessionFinished() when in TabbedNavigation mode

empty() is emitted so that MainWindow calls close() and we get rid of the
colours-flashing bug when closing the last session/tab in a window, see
commit bbec7225. However all that isn't relevant when we are in
konsole Part mode, where there are no tabs or MainWindows.

This fixes a crash in Dolphin when closing its window while the embedded
Konsole Part is open. The crash would happen as calling
ViewContainer::currentTabViewCount() would end up in the ViewSplitter code
path, which is no-op in Konsole Part.

To test:
- Open dolphin, make sure the terminal panel isn't open to begin with
- open the terminal panel
- close the dolphin window while the terminal panel is open

BUG: 441206
FIXED-IN: 21.08.1