🛑 Note: use Gorgious' solution, unless you really need Blender 3.0
In Blender 3.0 You can pass any non-zero value for a point, then use any of the 5 first outputs of the Attribute Statistic node:
This is only a slight improvement from Gorgious' answer, as it works for a single vertex as well. However, you could just use Scale and assume no object has scale = 0;0;0: