Trip: fix total_distance undercount

Previously Trip.total_distance returned None if any travel leg lacked a
distance, which led to undercounted trip stats (less than flight + train
totals).

Now it accumulates only distances that are present and ignores
missing/falsy values.  The method returns a float and yields 0.0 when no
distances are available.  Docstring updated accordingly.

Fixes #185

agenda/types.py

@@ -202,13 +202,18 @@ class Trip:
         """Countries flags for trip."""
         return "".join(c.flag for c in self.countries)
 
-    def total_distance(self) -> float | None:
-        """Total distance for trip."""
-        return (
-            sum(t["distance"] for t in self.travel)
-            if all(t.get("distance") for t in self.travel)
-            else None
-        )
+    def total_distance(self) -> float:
+        """Total distance for trip.
+
+        Sums distances for travel items where a distance value is present.
+        Ignores legs with missing or falsy distances rather than returning None.
+        """
+        total = 0.0
+        for t in self.travel:
+            distance = t.get("distance")
+            if distance:
+                total += distance
+        return total
 
     def total_co2_kg(self) -> float | None:
         """Total CO₂ for trip."""