Option to treat null values as error

Hey! First of all thanks for this great library. I hope it will end up in the material-ui core soon 🙂

I would like to be able to display an error when the date input is empty (when keyboard is true).

I see here that null value are excluded from errors.

One solution I have in mind would be to use the required prop of the TextField component to check this. If required is true, then we should return an error if date value is null.

Finally, we would need a requiredDateMessage for this.

If this makes sense to you, I can contribute this.

[dmtrKovalenko]

Yep, I think no sense in adding new prop required so the checking for required is a part of form-management like formik or redux-form. We just should dispatch onChange with proper value when the field is cleared

[ValentinH]

Calling onChange() is also fine for me.