[pvr] fix: directory update infinity loop if GetDirectory() return false

This fixes an infinity loop if `GetDirectory()` returns false and `GUIMediaWindow::Update()` wants to return to the previous directory. We override the directory path in each of the `Update()` method in all PVR related windows, so an update with an empty or different `strDirectory` param has no effect (except the recordings window).

I spotted this if I used the the pvr.demo addon which do not support timers and GetDirectory() return false.

@Jalle19 or @opdenkamp for review please.
@opdenkamp you can't override this in the derived classes, because you can't differ between this Update() and a normal Update() call, in both situations you get an empty dir path.