Either rearrange the furniture (and yes, chairs are easier to move than tables) or ask "is this seat taken?" and be prepared to eat with that stranger.
In this case, being prepared to eat with the stranger means respecting their personal space/that they got there first and are entitled to at least a quarter of the time, but not ignoring them if they start a conversation. Depending on my mood, if I tell someone that yes, the other seat is available, I might want to just dive back into my book or focus on my soup, or I might be in a mood to chat. I would be miffed if someone asked to sit at my table, ignored my overtures about the weather or activities in the area, and chatted with their companions. That would feel like an invasion in a way that neither sitting quietly nor including me in conversation would.