dnsdist: Fix handling of XFR requests over DoH

dnsdist: Fix handling of XFR requests over DoH

We did not properly handle incoming XFR requests received over DoH
When a TCP-only or DoT backend was configured, and the nghttp2 provider
used.
This commit fixes the assertion failure and makes sure that XFR
requests are denied with `NOTIMP` when received over DNS over HTTPS,
including DNS over HTTP/3. It also denies them when received over
DNS over QUIC as this is not properly handled at the moment, although
it does not cause a crash.