types: duration: Deal with empty as PT0S

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

Author: patrickelectric

xsd-types/src/types/duration.rs

@@ -97,6 +97,11 @@ impl FromStr for Duration {
 
         let mut dur: Duration = Default::default();
         let mut context = ParsingContext::new();
+
+        if s.is_empty() {
+            return Ok(dur);
+        }
+
         for (i, c) in s.chars().enumerate() {
             match c {
                 '-' => {