* fix warning as below:

...\libraries\ESP8266WiFi\src\WiFiServer.cpp: In member function 'WiFiClient WiFiServer::available(byte*)':
...\libraries\ESP8266WiFi\src\WiFiServer.cpp:120:55: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
  120 |             tcp_backlog_accepted(_unclaimed->getPCB());
      |                                                       ^