Fix project name when project-current returns nil

[fredericgiquel]

Thanks for implementing the request I made yesterday 👍

It works well except when you use project and you open a file in a directory outside a project. In this case, [nil] is displayed as in this screenshot:

This PR fixes this little problem.

[fredericgiquel]

It generates errors like: Error during redisplay: (eval (doom-modeline-segment--project-name) t) signaled (wrong-type-argument stringp nil)

So I close this PR.