Fix history.js null guard for speed and distance

formatSpeed and formatDistance return '—' on null/undefined values.
Server trips may have null avg/max speed if no speed data was recorded.

Co-authored-by: Copilot <[email protected]>

static/js/history.js

@@ -59,10 +59,12 @@
     }
 
     function formatDistance(kilometers) {
+        if (kilometers == null) return '—';
         return `${kilometers.toFixed(kilometers < 1 ? 2 : 1)} km`;
     }
 
     function formatSpeed(kilometersPerHour) {
+        if (kilometersPerHour == null) return '—';
         return `${kilometersPerHour.toFixed(1)} km/h`;
     }