Optional pages should be allowed to have a `value`

Currently, they can't:

https://github.com/inducer/relate/blob/e21c5ac9923e086de2703ce41e02e7e900f6140a/course/page/base.py#L829-L834=

There's an outright contradiction here, since the upload question spews a warning complaining about lack of `value`.

Start in course/page/base.py at the linked lines 829-834 and inspect how optional pages handle value. Compare that behavior with the upload question warning mentioned in the issue; done means optional pages can have a value without the contradictory lack-of-value warning.

Written by the indexing model from the issue text.