[DateInput] Selected value stays selected when canClearSelection=false with timePrecision enabled

[cmslewis]

Fixes #1737

Checklist

• Include tests

Changes proposed in this pull request:

BEFORE:

1. Make a DateInput with canClearSelection=false and timePrecision enabled.
2. Select a date.
3. Click the selected date again.
4. onChange is erroneously emitted with null, suggesting that the date has just been deselected, which shouldn't be possible.

AFTER:
DateInput no longer emits onChange with null when clicking selected date with canClearSelection=false with timePrecision enabled.

Reviewers should focus on:

Nothing in particular. Easy fix.

Screenshot

Here's a working example with the following props:

<DateInput
    canClearSelection={false}
    value={new Date(2017, Months.NOVEMBER, 6)}
    onChange={console.log}
    timePrecision={TimePickerPrecision.SECOND}
/>

[cmslewis]

Wrapped to 80 chars. @adidahiya is there a tslint rule or prettier rule that can enforce a different max-line-length for comments? Wrapping them at 120 chars makes them kind of hard to read.

[blueprint-bot]

Add uncontrolled test too

Preview: documentation
Coverage: core | datetime