Update onChange typing to include default type

[ebramanti]

As specified in the README, the default value for value is null, even though normally it will be of type Moment.

However, the onChange handler type is specified to always return a Moment:

onChange?: (newValue: Moment) => void;

I have updated the type to be a union type of Moment | null which properly handles the default value case returning from onChange.

Does any additional work need to be done to push these changes to @types/rc-time-picker? That is currently what I am using.

[vercel]

This pull request is being automatically deployed with ZEIT Now (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://zeit.co/react-component/time-picker/5jrszblkq
🌍 Preview: https://time-picker-git-fork-jadengore-fix-onchange-typing.react-component.now.sh

[codecov]

Codecov Report

Merging #181 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   91.86%   91.86%           
=======================================
  Files           6        6           
  Lines         381      381           
  Branches       99       99           
=======================================
  Hits          350      350           
  Misses         31       31

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e78a62e...12573a1. Read the comment docs.