DateRangeInput onChange's signature inaccurate

[matthieu-beteille]

The DateRangeInput can call its onChange handler will null values in the range (as mentioned in the following comment):

blueprint/packages/datetime/src/dateRangeInput.tsx

Lines 95 to 104 in 44043fd

	/**
	* Called when the user selects a day.
	* If no days are selected, it will pass `[null, null]`.
	* If a start date is selected but not an end date, it will pass `[selectedDate, null]`.
	* If both a start and end date are selected, it will pass `[startDate, endDate]`.
	*/
	onChange?: (selectedRange: DateRange) => void;
	/**

But the DateRange type doesn't include null in its signature:

blueprint/packages/datetime/src/common/dateUtils.ts

Line 19 in 44043fd

export type DateRange = [Date | undefined, Date | undefined];

It seems like throughout your codebase you use double equals to check for null/undefined values (value == null) so you wouldn't run into this issue.

But on stricter codebases which want to be more explicit, if we follow the types we would only check for === undefined when the value could also potentially be null.

[adidahiya]

This is true across virtually all of blueprint's type definitions because we don't use the strictNullChecks compiler option. Everything is potentially null. We'd like to enable this option, but it's a large change to make across the codebase. We'll get to it someday...