ls: Fix minor output mismatch

When a single directory is passed to ls in recursive mode, uutils ls
won't print the directory name
======================
GNU ls:
z:
======================
======================
uutils ls:
======================

This commit fixes this minor inconsistency and adds corresponding test.

tests/by-util/test_ls.rs

@@ -899,6 +899,12 @@ fn test_ls_recursive() {
 
     scene.ucmd().arg("a").succeeds();
     scene.ucmd().arg("a/a").succeeds();
+    scene
+        .ucmd()
+        .arg("z")
+        .arg("-R")
+        .succeeds()
+        .stdout_contains(&"z:");
     let result = scene
         .ucmd()
         .arg("--color=never")