Distinguish between update fody issue vs having no references

Currently Costura throws an error if the `ReferenceCopyLocalPaths` list is null. The error says to update Fody so that value is set.

The problem was Fody was up to date, but because I had no references that were copy local Fody was returning null for `ReferenceCopyLocalPaths` anyway.

This should return an empty list if there are no references. So now the states are:

- `ReferenceCopyLocalPaths == null` ERROR: Fody needs to be updated.
- `ReferenceCopyLocalPaths == empty list` There were no copy local references.
- `ReferenceCopyLocalPaths == list with items` The list of copy local references.