Which is the null set of the class of sets of open closed sets?

Question

I was working with the book Probability Theory: A comprehensive course, Achim Klenke. In the first chapter, page 4, they mention that the class of sets $ A = \{(a,b]:a,b\in \mathbb{R}, a \leq b \} $ is a semiring for $ \Omega = \mathbb{R}$, although not a ring. I was trying to prove this as an exercise, but I wasn't able to find the empty set, since in the case in which $ (a,b) = (0,0) $, we have that the set is open at the beginning in 0 but closed in 0 at the end, and I am not sure if this can be interpreted as the empty set.