fix: dead loop

[Reasno]

According to the document on wiki.swoole.com, the correct way to check disconnection is emtpy($buffer), instead of $buffer===false.

In our production environment, we periodically encounter infinite recv loops caused by the empty buffer. After the proposed change was made, the occasional dead loop went away.

[Yurunsoft]

if $buffer === '0', it's wrong

[Reasno]

if $buffer === '0', it's wrong

Then it should be $buffer == '' || $buffer === false

if $buffer === '0', it's wrong

maybe use $buffer === '' || $buffer === null || $buffer ===false

[Reasno]

The doc I referenced: https://wiki.swoole.com/#/coroutine_client/socket?id=%e5%ae%8c%e6%95%b4%e7%a4%ba%e4%be%8b