[Bug fix] When cache is empty, do not add file but initialize instead

Because adding a file will make the cache non-empty with that only file.
The next time a command like `projectile-find-file` is invoked, only
that single file is displayed. When the cache is empty, do not add file
but retrieves all project files first.

How to reproduce:

- Enter a project directory in Dired.
- Invalidate the cache.
- Open a file in project with Dired or anything, except `projectile-find-file`.
- After the file opened, run `projectile-find-file` and there's only one file in the list.
If you switch the `if` branch and the `else` branch you'll eliminate the need for both the `null` check and the `progn`.