Cleanup FileSystemLoader (Followup to #775)

[robfrawley]

This is a quick followup to #775.

The call to file_exists() is redundant and no longer required, as realpath() will already return false if the path does not exist.

Additionally, the call to ltrim() is no longer needed as realpath() will resolve the path properly, even if there are two slashes due to the root path and relative path concatenation.

Lastly, the ltrim() call may have also previously been a security consideration that would disallow the data_root being set to an empty string and absolute paths being passed as the relative path to find(). This is mitigated by the new check in the constructor that ensures data_root is not empty. A test for this edge-case is included.

(If someone really wants to push the boundaries of acceptable security, they can still set data_root to / and effectively pass absolute paths to the find() method, though this is a horrible idea... :-) )