Minor code refactoring after PR review.

TNL-836

openassessment/xblock/openassessmentblock.py

@@ -224,7 +224,7 @@ class OpenAssessmentBlock(
         See also: submissions.api for details.
 
         Args:
-            anonymous_user_id(str):
+            anonymous_user_id(str): A unique anonymous_user_id for (user, course) pair.
         Returns:
             (dict): The student item associated with this XBlock instance. This
                 includes the student id, item id, and course id.

openassessment/xblock/staff_info_mixin.py

@@ -234,6 +234,9 @@ class StaffInfoMixin(object):
         submission_uuid = None
         submission = None
         assessment_steps = self.assessment_steps
+        anonymous_user_id = None
+        submissions = None
+        student_item = None
 
         if student_username:
             anonymous_user_id = self.get_anonymous_user_id(student_username, self.course_id)