[libcalamaresui] Handle no-requirements-to-check case

- If there's no requirements at all, none of the watchers
   will call finished(), so do it once extra.

src/libcalamaresui/modulesystem/RequirementsChecker.cpp

@@ -92,6 +92,8 @@ RequirementsChecker::run()
         m_watchers.append( watcher );
         connect( watcher, &Watcher::finished, this, &RequirementsChecker::finished );
     }
+
+    QTimer::singleShot( 0, this, &RequirementsChecker::finished );
 }
 
 void