disable sending 'all white space' messages

[chmus]

Chat currently does not check whether the message is 'all white space' text. This small addition should fix it. However, I am currently unable to test if this will work, so it can be a wrong fix.

P. S. By the way, I am experienced neither in making pull requests nor in React 😅. Hopefully, it will not cause any problems.

[hakanrw]

Looks fine, but would be shorter and possibly faster if you used String.prototype.trim()