DateRangeInput: Controlled component doesn't accept value=[undefined, undefined]

[alshain]

Bug report

• Package version(s): core: 1.20.0, datetime 1.17.0
• Browser and OS versions: Chrome, Win 10

Steps to reproduce

render() {
        // As per documentation, unset date in uncontrolled mode be [null, null]
        // http://blueprintjs.com/docs/#datetime/daterangeinput
        // tslint:disable-next-line:no-any
        const value: [Date | undefined, Date | undefined] = [
            this.props.from.value ? this.props.from.value : undefined,
            this.props.to.value ? this.props.to.value : undefined
        ];
        return (
            <BlueprintDate.DateRangeInput
                onChange={this.onChange}
                value={value}
            />
        );

Actual behavior

The type definition requires value to be of the type [Date | undefined, Date | undefined]. Passing [undefined, undefined], however, results in the following:

Expected behavior

I expect this to produce the same behavior as [null, null], which is, as of now, disallowed by the type signature.