Is there any gchange can be state.graded but the percentage is None?

[This line](https://github.com/inducer/relate/blob/master/course/models.py#L1598) shows percentage can be `None`. If that's the case, [this line](https://github.com/inducer/relate/blob/master/course/models.py#L1753), and [this line](https://github.com/inducer/relate/blob/master/course/models.py#L1769) and [this line](https://github.com/inducer/relate/blob/master/course/models.py#L1778) will fail.

Read course/models.py at lines 1598, 1753, 1769, and 1778, then trace how a graded state can reach those paths. Determine whether percentage is guaranteed to be non-None; done means documenting that invariant or identifying the needed handling so those lines cannot fail.

Written by the indexing model from the issue text.