Just ignore the question and tell a story you wanted to tell anyway.

"Tell me about a time when you had a conflict with a coworker"

==

"I know you have some stories about great things you've done in your career. I want to hear those stories - that's the whole point of this interview - but I don't know what to ask, so I am giving you a very open-ended prompt, on the assumption that at least one of your stories included some kind of disagreement with a coworker."

edit to add: don't make something up. Besides dishonesty being generally bad for a variety of reasons, it's very likely that the interviewer is better at spotting liars than you are at lying.