Prevent any changes in the StorageSpoke if just going back

If user deselects all disks it means they just want to go back from the disk
selection screen. If that's the case, we shouldn't do any changes to storage
configuration.

Conflicts:
	pyanaconda/ui/gui/spokes/storage.py

(cherry-picked from commit 0c9a8bc)

pyanaconda/ui/gui/spokes/storage.py

@@ -858,6 +858,12 @@ def on_back_clicked(self, button):
             NormalSpoke.on_back_clicked(self, button)
             return
 
+        disks = [d for d in self.disks if d.name in self.selected_disks]
+        # No disks selected?  The user wants to back out of the storage spoke.
+        if not disks:
+            NormalSpoke.on_back_clicked(self, button)
+            return
+
         # Remove all non-existing devices if autopart was active when we last
         # refreshed.
         if self._previous_autopart:
@@ -867,12 +873,6 @@ def on_back_clicked(self, button):
         # hide/unhide disks as requested
         self._hide_unhide_disks()
 
-        disks = [d for d in self.disks if d.name in self.selected_disks]
-        # No disks selected?  The user wants to back out of the storage spoke.
-        if not disks:
-            NormalSpoke.on_back_clicked(self, button)
-            return
-
         # make sure no containers were split up by the user's disk selection
         self.clear_info()
         self.errors = checkDiskSelection(self.storage, self.selected_disks)