fix: initialise reasonForChange default to empty string

The basic details form became dirty right after opening edit mode in react strict mode.

This happened because reasonForChange started as undefined, but the textarea used an empty string.
React Hook Form saw that as a change and marked the form dirty.

Set reasonForChange default to empty string so the form starts clean and no false unsaved-changes warning appears.

Author: patricvincit

ui/src/components/stop-registry/stops/stop-details/basic-details/BasicDetailsSection.tsx

@@ -27,6 +27,7 @@ const mapStopBasicDetailsDataToFormState = (stop: StopWithDetails) => {
     timingPlaceId: stop.quay?.timingPlaceId ?? stop.timing_place_id ?? null,
     stopState: stop.quay?.stopState ?? undefined,
     stopTypes: stop.quay?.stopType,
+    reasonForChange: '',
   };
 
   return formState;