I have been on both sides....as student and as instructor.
As a student, when Sensei and I were the only people on the mat, out of respect to Sensei, I asked him if he would rather forgo class that evening. As it was a very infrequest occurance, I did not see it as finacial loss.
Indeed, I would take the opportunity to discuss an area I was lacking....most areas!!
and also to do some dojo cleaning.
As an instructor, if the student wished to practice, then practice we did. I learn something each time I step on the mat as student or instructor. So I find a class of 2 or a class of 22 is always a learning experience.
But to the question...is Sensei obligated to teach to a class of one? Much Aikido is learned from the fine art of "carefully watching". It is very difficult to see what Sensei is doing as you are flying through the air. If a class of one occurs often, then that is an expected class size and Sensei should teach. If a class of one happens once or twice a year.....give Sensei a break...and give her/him a Heineken