Handle empty session files gracefully

There's still much more to do for #1926, but with this we at least handle one
common case gracefully.

No tests because test_sessions.py is still in a kinda bad place...

doc/changelog.asciidoc

@@ -75,6 +75,7 @@ Fixed
 - When no documentation has been generated, the plaintext documentation now can
   be shown for more files such as `qute://help/userscripts.html`.
 - Crash when doing initial run on Wayland without XWayland
+- Crash when trying to load an empty session file.
 
 Removed
 ~~~~~~~

qutebrowser/misc/sessions.py

@@ -414,6 +414,9 @@ class SessionManager(QObject):
             raise SessionError(e)
 
         log.sessions.debug("Loading session {} from {}...".format(name, path))
+        if data is None:
+            raise SessionError("Got empty session file")
+
         for win in data['windows']:
             window = mainwindow.MainWindow(geometry=win['geometry'],
                                            private=win.get('private', None))