Cannot replicate first type stability example

[BSchilperoort]

https://carpentries-incubator.github.io/efficient-computing-in-julia/100-introduction-performance.html#type-stability

In this case we may observe that the induced type is Union{Nothing, T}.

How may we observe this? I don't see it.

If we run @code_warntype we can see the yellow highlighting of the union type

I don't see the highlighting or that union type...

Using Julia 1.12