fix: exception on null embedded scalar values

Author: efrederickson

mongoengine/queryset/base.py

@@ -2036,6 +2036,8 @@ def lookup(obj, name):
             chunks = name.split("__")
             for chunk in chunks:
                 obj = getattr(obj, chunk)
+                if obj is None:
+                    break
             return obj
 
         data = [lookup(doc, n) for n in self._scalar]