Fix for projects in groups with no physical folder

[mike-at-home]

When placing project references in nested groups with no physical folder, the derived path would end up being absolute. (it would start with "/") because ["", ""].joined(separator: "/") results in "/" (root).

By filtering out empty locations before joining, this problem is avoided. There might be a better approach. (This PR serves dual purpose as issue report and suggested fix)

[ileitch]

Thanks!