At anything approaching relativistic speeds, the name for what would happen when those random atoms of mostly hydrogen impacted the spacecraft's hull would not be friction, but more likely, fusion, the formation of a high energy plasma and a bit of radiation that used to be that bit of hull. Can't imagine that ending well.

However, since the hypothetical warp drive doesn't actually push the ship through space, but bend space around the ship, there should be no contact between the ship and "space air" in the path of travel.