Optimize print_server_started() to avoid throw an exception when using the QUIC protocol.

[Jonney]

When using the QUIC protocol, an exception will be thrown:

(python-3.12.8) root@server:/root/python-proxy# pproxy -v -l quic+http://127.0.0.1:8080 --ssl ssl.crt,ssl.key Using uvloop Serving on 127.0.0.1:8080 by http Start server failed. ==> 'QuicServer' object has no attribute 'sockets'