The codegen path for map-of-slice-of-pointer fields, skipped
nil-valued entries. That dropped the key from the map.
This broke how dns.Config.Routes uses nil values sentinels.
Fixes#19730Fixes#19732Fixes#19746Fixes#19744
Change-Id: Ic6400227f4ab21b3ca0e8c0eeecf9b83d145a9ab
Signed-off-by: Fernando Serboncini <fserb@tailscale.com>