BUG/MINOR: check: fix QUIC check test when QUIC disabled

BUG/MINOR: check: fix QUIC check test when QUIC disabled

Latest commit ef206d441c ("MINOR: check: ensure QUIC checks configuration
coherency") introduced a regression when QUIC is not compiled in. Indeed,
not specifying a check proto sets mux_proto to NULL, which also happens to
be the value of get_mux_proto("QUIC"), so it complains about QUIC. Let's
add a non-null check in addition to this.

No backport is needed.