BUG/MINOR: quic: fix crash on handshake io-cb for null next enc level

BUG/MINOR: quic: fix crash on handshake io-cb for null next enc level

When arriving at the handshake completion, next encryption level will be
null on quic_conn_io_cb(). Thus this must be check this before
dereferencing it via qc_need_sending() to prevent a crash.

This was reproduced quickly when browsing over a local nextcloud
instance through QUIC with firefox.

This has been introduced in the current dev with quic-conn Tx
refactoring. No need to backport it.