nautilus_file_peek_display_name: Don't return NULL

If the name is the empty string then nautilus_file_set_display_name
won't actually set the display name. In this case we were returning
NULL from nautilus_file_peek_display_name, which some of our callers
weren't prepared to handle. This led to crashes.

https://bugzilla.gnome.org/show_bug.cgi?id=700507