fix: jump_to should redirect to first unit on invalid key

Previously, it would 404. While accurate, it's not a great user
experience. Users can be offered invalid jump_to paths in the normal
course of things, if course content disappears or they lose access
to it.

In both cases, they might be offered a resume URL in the courseware
that would be to a now-invalid location.

With this change, that invalid link will at least give them
*something* (the first unit in the course) rather than an error
page.

This also (unrelatedly) fixes an exception when the learning MFE
outline page tries to render a course that contains sequences
with no children.

AA-867