Hotfix: Handle null point overrides in submission comments

This fixes the below production error:
```
  File ".../tin/tin/apps/submissions/models.py", line 136, in point_override
    return sum(c.point_override for c in self.comments.all())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
```

tin/apps/submissions/models.py

@@ -133,7 +133,7 @@ def points_possible(self):
 
     @property
     def point_override(self):
-        return sum(c.point_override for c in self.comments.all())
+        return sum(c.point_override for c in self.comments.all() if c.point_override)
 
     @property
     def points(self):