cube-seal: prefer is_empty() (clippy)

crates/cube-kernel/src/bin/cube-seal.rs

@@ -73,7 +73,7 @@ fn main() {
         .and_then(|v| v.parse().ok())
         .unwrap_or(5000);
     let positional: Vec<&String> = args.iter().filter(|a| !a.starts_with("--")).collect();
-    if positional.len() < 1 {
+    if positional.is_empty() {
         eprintln!("usage: cube-seal <list-file> [<key-file>] [--dry-run] [--open-check] [--sync-every N]");
         std::process::exit(2);
     }