fix: disable relay if built-in DERP is disabled (#12654)

Fixes https://github.com/coder/coder/issues/12493

enterprise/replicasync/replicasync.go

@@ -17,7 +17,6 @@ import (
 	"golang.org/x/xerrors"
 
 	"cdr.dev/slog"
-
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/cliutil"
 	"github.com/coder/coder/v2/coderd/database"
@@ -38,8 +37,9 @@ type Options struct {
 	TLSConfig       *tls.Config
 }
 
-// New registers the replica with the database and periodically updates to ensure
-// it's healthy. It contacts all other alive replicas to ensure they are reachable.
+// New registers the replica with the database and periodically updates to
+// ensure it's healthy. It contacts all other alive replicas to ensure they are
+// reachable.
 func New(ctx context.Context, logger slog.Logger, db database.Store, ps pubsub.Pubsub, options *Options) (*Manager, error) {
 	if options == nil {
 		options = &Options{}
@@ -255,6 +255,13 @@ func (m *Manager) syncReplicas(ctx context.Context) error {
 		if replica.ID == m.id {
 			continue
 		}
+		// Don't peer with nodes that have an empty relay address.
+		if replica.RelayAddress == "" {
+			m.logger.Debug(ctx, "peer doesn't have an address, skipping",
+				slog.F("replica_hostname", replica.Hostname),
+			)
+			continue
+		}
 		m.peers = append(m.peers, replica)
 	}
 	m.mutex.Unlock()