Blank screen when booting in debug mode

[Frédéric Fauberteau <triaxx@triaxx.online>]

I am following this guide to try to debug a NetBSD kernel: 
https://www.netbsd.org/docs/kernel/kgdb.html

I have compiled a KGDB kernel for which the diff with GENERIC is only:
$ diff -U0 sys/arch/amd64/conf/GENERIC sys/arch/amd64/conf/KGDB
--- sys/arch/amd64/conf/GENERIC 2024-10-27 06:58:47.853098703 +0100
+++ sys/arch/amd64/conf/KGDB    2025-02-12 23:05:49.228411666 +0100
@@ -126 +126 @@
-options        DDB             # in-kernel debugger
+#options       DDB             # in-kernel debugger
@@ -129,3 +129,3 @@
-options        DDB_HISTORY_SIZE=512    # enable history editing in DDB
-#options       KGDB            # remote debugger
-#options       KGDB_DEVNAME="\"com\"",KGDB_DEVADDR=0x3f8,KGDB_DEVRATE=9600
+#options       DDB_HISTORY_SIZE=512    # enable history editing in DDB
+options        KGDB            # remote debugger
+options        KGDB_DEVNAME="\"com\"",KGDB_DEVADDR=0x3f8,KGDB_DEVRATE=9600
@@ -133 +133 @@
-options DDB_COMMANDONENTER="trace;show registers"
+#options DDB_COMMANDONENTER="trace;show registers"

I have installed it:
# cp -v sys/arch/amd64/compile/obj/KGDB/netbsd /netbsd-KGDB

I booted this kernel by adding the following line in boot.cfg:
menu=Boot NetBSD 10.99.12 (KGDB):rndseed /var/db/entropy-file;root 
NAME=rootdev;boot NAME=rootdev:netbsd-KGDB -d

Whether the null modem cable is plugged in or not, I have a blank screen 
after the memory addresses finish to scroll.

The kernel boots correctly without -d option.