Exactly. And in many languages that is quite difficult to ensure at compile time. In java all object parameters are by definition optoinal. Same thing in C# (before C#8 brought non-nullable types).
Obviously in F# or Haskell or C#8 or Rust this isn't a problem. But what we are talking about is: how do you handle, at runtime, in a language such as C#7 or Java, an null parameter being passed to a method that must return a value based on its parameter, but was passed null? It's a choice between very bad options where the least bad is usually to throw an ArgumentNullException or similar. Because there is nothing better to do.
Obviously in F# or Haskell or C#8 or Rust this isn't a problem. But what we are talking about is: how do you handle, at runtime, in a language such as C#7 or Java, an null parameter being passed to a method that must return a value based on its parameter, but was passed null? It's a choice between very bad options where the least bad is usually to throw an ArgumentNullException or similar. Because there is nothing better to do.