All the components should tell the differences of undefined and null

[benjycui]

It is confusing now, some components need undefined when we want to clear value of a property, but others need null.

We should unify the behaviors of all the components, and they should:

1. Treat undefined as undefined, even if developers pass undefined to components explicit
2. Treat null as defined, and it means clear the value of a property

This will be a breaking change, so we should do this in next major version.

cc @warmhug

[paranoidjk]

+1 for this

[otakustay]

+1, undefined should mean "a unintentional empty value" so it could be a hint of a future error, leave null as the behavior of intentional clear value

[afc163]

This had been resolved in antd v4.