If you say no, most interviewers will keep drilling deeper to find a conflict. The key is how you behaviorally reacted to conflict and what you did to resolve it.

Example:

"Yes, I have had conflicts in the past. Never major ones, but there have been disagreements that needed to be resolved. I have found that when conflict occurs, it helps to fully understand the other person's perspective, so I take time to listen to their point of view, and then I seek to work out a collaborative solution.