Bosun's statement needs to be qualified a bit. While it's true you can have an Action without an ActionForm defined for it, you cannot specify such an action in an <html:form> tag. You can only call such an action as URL (e.g. myAction.do).
One option would be to use the plain HTML <form> tag specifying "myAction.do" as the action. If you still want to use the <html:form> tag but don't want to create an Action form, simply declare the DynaActionForm class as the Actionform.