In sbe::ScanStage, there is some logic for dealing with the case where _coll is nullptr, though it appears to be the case that we always throw an exception before we reach these cases. We should remove this dead code, and similar elsewhere in the file and replace it with a MONGO_UNREACHABLE_TASSERT where appropriate.

As part of this work we should also see if the analogous code in ix_scan.cpp is dead as well.