Emit onChange with null when DateInput is empty

[leebyp]

Fixes #504, Fixes #546

Changes proposed in this pull request:

• Return null as the date argument in DateInput's onChange handler

[blueprint-bot]

emit onchange with null when date is empty

Preview: docs
Coverage: core | datetime

[giladgray]

can we get a unit test? 🥁

[blueprint-bot]

fix linting

Preview: docs
Coverage: core | datetime

[blueprint-bot]

unify interactions

Preview: docs
Coverage: core | datetime

[blueprint-bot]

fix linting

Preview: docs
Coverage: core | datetime

[giladgray]

would've liked the tests refactor as a separate PR but oh well, looks good!

[adidahiya]

can we have it return undefined instead of null? I would really like to avoid introducing null at the API boundaries that we control.

[giladgray]

@adidahiya i was wondering the same thing! ☁️

DateRange uses null to mean "no date", though its typings declare Date | undefined. someone is definitely wrong. we should fix this whole thing at once, not in this PR.

i think it should wait for strictNullChecks #325 because that'll make it so much easier to test.

[leebyp]

i mainly followed the convention from DatePicker but yup, all three should be doing the same thing whichever we decide. I prefer null because undefined is already tied to the concept on "uncontrolled components" in the BP world.