`doom doctor` throws a type error for enabled modules without a
doctor.el and/or packages.el file. This occurs because, in
in775ee2f04aad, `doom-module-expand-path` was changed to return nil if
the target file did not exist, and the doctor wasn't updated to handle
this. It passes `nil` to `load`, causing this type error.
Amend: 775ee2f04a