Fix folder opening in ActivityListModel

Fix folder opening in ActivityListModel

After fixing the crash in the previous commit, double-clicking on Activity list rows still didn't work.

This fix partly reverts commit 8546d53b05a76271e245f085d1caa5d405dadb8d in ActivityItemDelegate::PathRole
of ActivityListModel::data, but adds a new check for relPath's existence in line 74.

I'm assuming the previous change there has been done to shorten the code and avoid opening the user's home
folder upon clicking entries which file value is "App Password". The new path-check takes care of that too now.

Signed-off-by: Michael Schuster <michael@schuster.ms>