Usually when you assume a pin or roller connection at a joint, it is not actually going to be a "true" pin or roller in reality. There's going to be some sort of fixity or resistance in almost any connection because it is integral to an entire system that can resist forces in ways that engineers don't account for in order to simplify design. In this case you designed that connection as a roller, therefore assuming that there was no horizontal reaction at that connection. This is a correct assumption in this situation since your stringer is connected into a wide flange beam where any horizontal force on the beam is acting in the weak axis, where the beam will have little stiffness to counteract this force. If you designed the stringer with adequate stiffness, meaning it is within deflection limits, the horizontal deflection at the joint will be minimal. The connection shown is fine for this application, if you want to be sure about it you can calculate the horizontal deflection at the stringer/beam joint and make sure the beam won't yield under that deflection.