cmd/derper: support no mesh key (#14931)

Incorrect disabled support for not having a mesh key in
d5316a4fbb

Allow for no mesh key to be set.

Fixes #14928

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

cmd/derper/derper.go

@@ -236,6 +236,8 @@ func main() {
 
 	if meshKey == "" && *dev {
 		log.Printf("No mesh key configured for --dev mode")
+	} else if meshKey == "" {
+		log.Printf("No mesh key configured")
 	} else if key, err := checkMeshKey(meshKey); err != nil {
 		log.Fatalf("invalid mesh key: %v", err)
 	} else {