fix: add null safeguard and source-asymmetry test for buildVehiclePopupData

[VishalRaut2106]

This PR addresses the two follow-up suggestions from the maintainer regarding buildVehiclePopupData edge cases.

Changes

1. Null Safeguard: Added optional chaining (activeTrip?.tripHeadsign) inside buildVehiclePopupData so that if activeTripMap.get(...) ever returns undefined, the code gracefully returns undefined instead of throwing a fatal TypeError.
2. Asymmetry Test: Added a new test in vehicleUtils.test.js where vehicle and activeTrip have identically-named properties with different values. This explicitly proves that the helper is pulling data from the correct source object.
3. Null-State Test: Added a test that passes undefined for activeTrip to guarantee the optional chaining correctly prevents crashes.

Closes #508

[coveralls]

Coverage is 81.05% — VishalRaut2106:fix/vehicleUtils-null-safeguard-508 into OneBusAway:develop. No base build found for OneBusAway:develop.