fix clippy::comparison_to_empty in tests

Author: hellow554

tests/test_std.rs

@@ -1143,7 +1143,7 @@ fn tree_fold1() {
         "0 1 x 2 3 x x 4 5 x 6 7 x x x 8 9 x 10 11 x x 12 13 x 14 15 x x x x",
     ];
     for (i, &s) in x.iter().enumerate() {
-        let expected = if s == "" { None } else { Some(s.to_string()) };
+        let expected = if s.is_empty() { None } else { Some(s.to_string()) };
         let num_strings = (0..i).map(|x| x.to_string());
         let actual = num_strings.tree_fold1(|a, b| format!("{} {} x", a, b));
         assert_eq!(actual, expected);