Avoid star error messsage when no changed files

secondary.py

@@ -57,6 +57,8 @@ class Backup:
                                         cwd=os.path.join(mount, path),
                                         stdin=self.primary_out)
         atexit.register(cond_kill, self.extract)
+        # Make sure that the generated star archive is not empty
+        self.primary_in.write(b'.\n')
 
     def close(self):
         self.primary_in.flush()