types: duration: Add test for empty string and PT0S

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Author: patrickelectric

xsd-types/src/types/duration.rs

@@ -301,6 +301,8 @@ mod tests {
 
     #[test]
     fn duration_parsing_test() {
+        assert_eq!(Duration::from_str(""), Duration::from_str("PT0S"));
+        check_valid("PT0S");
         check_valid("P2Y6M5DT12H35M30S");
         check_valid("P1DT2H");
         check_valid("P20M");