Make unnecessary arguments optional

[IlyaSemenov]

Please make the following arguments optional:

• onClosed
• onEndReached

Right now user must pass empty handlers such as onClosed={() => {}}.

[issue-label-bot]

Issue-Label Bot is automatically applying the label feature_request to this issue, with a confidence of 0.76. Please mark this comment with 👍 or 👎 to give our bot feedback!

Links: app homepage, dashboard and code for this bot.

[IlyaSemenov]

Also, I don't see how items[x].Value is ever used, so it doesn't have to be required, too.