allow users to clear the default context for queries

[dpetran]

This is a partial fix for #395, allowing users to supply an explicitly nil context to clear the default context.

This is just the bare minimum changes necessary to support the new behavior of the json-ld dependency.

[bplatz]

With the approach taken with PR #437, now the default behavior is to always clear default context. I don't think this PR is needed as it stands, but perhaps parts of it should still be included.

[dpetran]

We still need this for correct context handling (if someone passes us a bunch of contexts interspersed with nils, we'll need to handle that case) and for a fix to #395, I think. I'll try to see how to rebase on top of the existing context work.

[bplatz]

We still need this for correct context handling (if someone passes us a bunch of contexts interspersed with nils, we'll need to handle that case) and for a fix to #395, I think. I'll try to see how to rebase on top of the existing context work.

Sure, just would say its not high priority if it take any amount of work. If someone doesn't want to include a context, they can simply not include it. Before with our default context people didn't have that option, so it was needed. But now they have that option so it isn't a significant issue short-term.

[dpetran]

Well, I just tried using the json-ld from fluree/json-ld#14 directly on main and it didn't break any tests, so I don't think this PR is needed anymore, but I do think we should merge that other one. That would solve #395 at least.