internal: failback with self-signed cert if cert for tenant fails to load

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

internal/web/tenant_tls/tenant_tls.go

@@ -78,5 +78,8 @@ func (w *Watcher) GetCertificate(ch *tls.ClientHelloInfo) (*tls.Certificate, err
 		return w.fallback, nil
 	}
 	cert := w.cs.Get(*bestSelection.WebCertificate.Get())
+	if cert == nil {
+		return w.fallback, nil
+	}
 	return cert, nil
 }