Handle NONE and NOASSERTION relationships in SPDX

[ctron]

Haven't seen them so far. However, technically it is possible to have a relationship with the target NONE or NOASSERTION. NONE seems simple, the latter I have no idea what to do with.

https://spdx.github.io/spdx-spec/v2.3/relationships-between-SPDX-elements/#1111-description

In cases where there are "known unknowns", the use of the keyword NOASSERTION can be used on the right hand side of a relationship to indicate that the author is not asserting whether there are other SPDX elements (package/file/snippet) that are connected by relationships or not. That is, there could be some, but the author is not asserting one way or another.

Similarly, the use of the keyword NONE can be used to indicate that an SPDX element (package/file/snippet) has no other elements connected by some relationship to it.

[ctron]

I guess we have an example now with #762