The typechecker would only allow on the union operations which are allowed on both types, anything beyond that would first have to use type checks or assertions in order to split the union.
So, again, what are you talking about?
There are some exceptions to the behaviour you describe, like for instance with C# which for the longest time only allowed value types to be annotated as nullable, and only very recently extended this to reference types, and only as an opt-in feature, and the type checker only throws warnings, etc. That would be a case of a language which is not "null safe" but provides faculties for accomplishing that.