Source files that are right next to the project file result in an
empty "object subdirectory" if object_parallel_to_source is set.
We must not attempt to create empty directories.
Fixes: QTBUG-81271
Change-Id: I431f9fbe46f50fbbaa5d6a59966bfb059418036c
Reviewed-by: Kai Koehne <kai.koehne@qt.io>