Make code of drupal_realpath() match docs.

Breaking this off from #1191236: Correct file_copy, file_move, file_save_data, and drupal_realpath to match documentation and their unmanaged counterparts.

The documentation for the $uri parameter to drupal_realpath() says, in part:

If this value is omitted, Drupal's default files directory will be used [public://].

However, this claim is untrue. Omitting the $uri parameter will actually result in a PHP warning, and the function will return FALSE.