allow document.asset.copyright to be an empty string

[sdumetz]

Deleting document.asset.copyright in voyager-story makes for a document that fails to validate :

1. Because document.asset.copyright has a minLengthof 1 but is optional in document.schema.json
2. Because CVDocument.ins.copyright.value is always serialized, even when it is an empty string, which it can be since Keep Copyright notices #285 (well done, me! 😉 )

Both are reasonable, but can't work together.

At first I thought about saving copyright only when not empty but then Voyager will always revert back to the default Smithsonian notice on reload (and then will write it on next save), which is not ideal.

So I removed the minLength requirement and marked the property as required to reflect the behavior.