Not possible to rename a project source if it wasn't originally assigned a path

[alex-ketch]

Attempting to rename a project source fails if it wasn't assigned a path at the time of creation.

To reproduce:

1. Link a new GitHub source to a project. e.g. https://github.com/stencila/hub/blob/master/README.md
2. ⚠️ Leave the Path input field blank
3. Wait for source to be pulled
4. Go to Project Sources list, and click Rename in the source action menu
5. Give the source a new path, and click Rename
6. 🐛 Nothing happens