Resolve manual_string_new pedantic clippy lint in test

    error: empty String is being created manually
        --> tests/test.rs:1110:18
         |
    1110 |         ("\"\"", "".to_string()),
         |                  ^^^^^^^^^^^^^^ help: consider using: `String::new()`
         |
         = note: `-D clippy::manual-string-new` implied by `-D clippy::pedantic`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_string_new

tests/test.rs

@@ -1107,7 +1107,7 @@ fn test_parse_string() {
     ]);
 
     test_parse_ok(vec![
-        ("\"\"", "".to_string()),
+        ("\"\"", String::new()),
         ("\"foo\"", "foo".to_string()),
         (" \"foo\" ", "foo".to_string()),
         ("\"\\\"\"", "\"".to_string()),