add back ability to infer fusion type

[katiestahl]

cancervariants/fusion-curation#314 (comment)

I think from #174, fusion types can no longer be inferred if its elements are missing any types for the fusion elements. We should consider adding validators/some sort of way to add this inference back, if needed? I'm not entirely sure that there's ever a possible case where this would be necessary, but it wouldn't be bad to have.

[jsstevenson]

Yeah, I don't think this needs to be a high priority. Pretty sure we just included it because we could. If it doesn't get fixed we can let the stalebot eat it.

[katiestahl]

Yeah, I don't think this needs to be a high priority. Pretty sure we just included it because we could. If it doesn't get fixed we can let the stalebot eat it.

sounds good to me!