Clean compiler warning

```
warning: length comparison to zero
   --> src/lib.rs:203:27
    |
203 |             let leaf = if prefix.len() > 0 {
    |                           ^^^^^^^^^^^^^^^^ help: using `is_empty` is more concise: `!prefix.is_empty()`
    |
    = note: #[warn(len_zero)] on by default
    = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#len_zero
```

Author: opilar

src/lib.rs

@@ -200,7 +200,7 @@ impl TempDir {
         let mut rng = thread_rng();
         for _ in 0..NUM_RETRIES {
             let suffix: String = rng.gen_ascii_chars().take(NUM_RAND_CHARS).collect();
-            let leaf = if prefix.len() > 0 {
+            let leaf = if !prefix.is_empty() {
                 format!("{}.{}", prefix, suffix)
             } else {
                 // If we're given an empty string for a prefix, then creating a