Return an empty dict if physics package missing

This is needed to work properly with `update_physics_package()`

Signed-off-by: Patrick Avery <[email protected]>

hexrdgui/hexrd_config.py

@@ -3049,7 +3049,7 @@ def apply_absorption_correction(self, v):
     @property
     def physics_package_dictified(self):
         if not self.has_physics_package:
-            return None
+            return {}
 
         return self.physics_package.serialize()