Handle missing optional matches

The Index trait for Captures panics if there is no match at a given
index.

Closes #193

src/operator/parse.rs

@@ -34,9 +34,12 @@ impl Parse {
             None => Ok(None),
             Some(capture) => {
                 let mut values: Vec<data::Value> = Vec::with_capacity(self.fields.len());
-                for i in 0..self.fields.len() {
+                for item in capture.iter().skip(1) {
                     // the first capture is the entire string
-                    values.push(data::Value::from_string(&capture[i + 1]));
+                    match item {
+                        None => values.push(data::Value::None),
+                        Some(match_) => values.push(data::Value::from_string(match_.as_str())),
+                    };
                 }
                 Ok(Some(values))
             }

tests/structured_tests/missing_optional_match.toml

@@ -0,0 +1,7 @@
+query = """* |parse regex "(?P<url>.+)(?P<query>\\?.+)?""""
+input = """/track/?verbose=1&ip=1&_=1716389413340"""
+output = """
+[query=None]         [url=/track/?verbose=1&ip=1&_=1716389413340]
+"""
+error = """
+"""