chgrp: don't output an empty error

tests/by-util/test_chgrp.rs

@@ -228,3 +228,19 @@ fn test_big_h() {
         );
     }
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn basic_succeeds() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let one_group = nix::unistd::getgroups().unwrap();
+    // if there are no groups we can't run this test.
+    if let Some(group) = one_group.first() {
+        at.touch("f1");
+        ucmd.arg(group.as_raw().to_string())
+            .arg("f1")
+            .succeeds()
+            .no_stdout()
+            .no_stderr();
+    }
+}