Make URN a non-Nullable field

[bencap]

(at least) the score set URN is currently a nullable field, even though it is de-facto required on all DB records. This causes a minor annoyance while type checking, since mypy presumes this field has the potential to by Nonetype.

As part of this fix, we should make sure other URN fields are non-nullable and that any associated type hints are corrected to eliminate the Optional keyword.