My intention was to handle None as to mean 'don't provide a link to this'. I see that I only did this in one of the two callers.

After I implemented this, I spoke to Marco about the fact that we have some xblocks that we can't link to, and he changed the design so that we could never have a breadcrumb linking to a subsection (at least right now). So, the None checks are no longer strictly necessary, so I could switch it to an exception if you'd like. Going forward, I think we'll need to have a valid URL for every xblock, so we can build a generic outline that can click through to any item.