Having a hook plugin, struts actions returning a null value cannot be overridden. The reason for this is a NullPointerException in the execute() -method in com.liferay.portal.struts.StrutsActionAdapter. In the current version (LR6 EE SP2, 6.0.12) the method expects the original action to return an ActionForward as seen below.

@Override
public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception {

ActionForward actionForward = _action.execute(_actionMapping, _actionForm, request, response);
return actionForward.getPath();

}

An example of a struts action returning null is com.liferay.portal.action.LoginAction (/c/portal/login).