Allow using send button when there is no message text

In my app sometimes a user can attach a file, then push send, without typing a message. I can use `sendDisabled` to make the button become enabled when there is an attachment, but clicking it does not trigger the `onSend` handler because of https://github.com/chatscope/chat-ui-kit-react/blob/master/src/components/MessageInput/MessageInput.jsx#L134

I think this check should be replaced with a sendDisabled check, or no check since the button disablement should be enough to prevent this from being called when we don't want it.