Allow setup without shared folders/files

When setting shared folders/files to an empty array, or not setting the variables at all, `mina setup` will fail with the following:

```
$ mina setup
-----> Setting up /some/directory/
       bash: -c: line 4: syntax error near unexpected token `&&'
       bash: -c: line 4: `(cd /some/directory/shared &&  && cd -)'
       Connection to hostname.tld closed.

 !     Run Error
```