Return false if there isn't a latest revision

Resolved PHP debug errors when on the dashboard Add New Document screen.  

Errors:
Warning: Cannot modify header information - headers already sent by (output started at /plugins/wp-document-revisions/wp-document-revisions.php:372) in /wp-includes/option.php on line 568
Notice: Trying to get property of non-object in /plugins/wp-document-revisions/wp-document-revisions.php on line 372 
Merge pull request #42 from rachelbaker/master

Adding a new document won't have a file attached yet, props @rachelbaker