Package concluded license: Allow using "NONE"?!

[fviernau]

@sschuberth @tsteenbe @mnonnenmacher : Shall we file a ticket for making "NONE" a valid value for the package concluded license in ORT?

Note that for license finding curations we already do allow that.

Originally posted by @fviernau in https://github.com/oss-review-toolkit/ort/pull/3117#discussion_r494196928

I'm a bit undecided. On the one hand it would be consistent to align on SPDX expressions for "everything about licenses". On the other hand, checking for (not) null is much more convenient than checking for isPresent() / isNotPresent(). My hunch is that we should still align on SPDX expression, but I'd like to also hear @mnonnenmacher opinion on that once he's back.

Originally posted by @sschuberth in https://github.com/oss-review-toolkit/ort/pull/3117#discussion_r494258854

[tsteenbe]

@fviernau I prefer to follow SPDX see https://spdx.github.io/spdx-spec/3-package-information/#313-concluded-license: Data Format: SPDX License Expression | NONE | NOASSERTION

[sschuberth]

Very much related to https://github.com/oss-review-toolkit/ort/issues/2852 for declared licenses.