If someone truly gives you no energy, than stay out of the way. If they wish to pursue, they'll come forward, sending some momentum forward. If someone can take an immovable stance with no openings while walking-- see if they can do it while running as you head for the exit... I think it's not important what technique you do, as long as you finish it. At my dojo, if you should "fudge" a technique, it's best to do something else. As one of my sensei said "if his attack gives you iriminage, perform iriminage. If his attack gives you ikkyo, do ikkyo." I'd say it really all depends on ma-ai and re-ai... if your timing or distancing won't allow you to do a technique, just do something else... don't think, just act... mushin, neh?