warn the null / undefined is not the expected type even if the prop is not required and no default value

[TerenceZ]

For the example followed, when I create the component without providing the f property, it will warn that the null / undefined is not the expected type. I think it should only warn when the property is required or exist an input value but not of the expected type.

Vue.component('comp', {
    props: {
        f: { type: Function }
    }
});