fix(routes-compute-routes): resolve API validation errors on optional dropdowns

[willum070]

What this PR does

Fixes an InvalidValueError thrown by the Routes API in the routes-compute-routes sample when submitting the form without selecting optional preferences.

Root Cause

In a previous chore update (#1574), new ESLint rules (@typescript-eslint/prefer-nullish-coalescing) prompted a switch from logical OR (||) to nullish coalescing (??) for handling optional <select> inputs.

Because FormData.get() returns an empty string ("") for default unselected options, the nullish coalescing operator ("" ?? undefined) evaluated to "" instead of undefined. This sent an invalid empty string value to the Maps API for enum fields, causing the validation error.

The Fix

Replaced the ?? fallback logic with explicit ternary operators (value === '' ? undefined : value) for the following optional fields:

• routingPreference
• polylineQuality
• transitPreference.routingPreference

This ensures that empty strings correctly fall back to undefined before being sent to the API, while still satisfying the strict ESLint rules that prohibit the legacy || pattern.

Change-Id: Ifa79159c75cd2da592071c0d7cb4b20975fb5a8e