Returns a function that combines two values stored in vectors into a new vector value. Although generic, this function will only be called with the `T` set to the type of vector that is being built. Since `VectorConstruction` is not generic, the type cannot be statically propagated.

Assuming `*` is the result of `GetFunction`, this property returns true when for all `x` it is the case that `Missing * x = x = x * Missing`. This enables certain optimizations (as we do not have to call `*` when one argument is N/A)