front: Update fetchSubregions to return empty array.

Modified fetchSubregions function to return an empty array instead of null when
no subregions are available, aligning with the expected return type and
ensuring consistency in handling empty results across the application.

Issue: #189

Signed-off-by: Nikolay Martyanov <ohmspectator@gmail.com>

frontend/src/api/index.js

@@ -34,8 +34,7 @@ export const fetchSubregions = async (regionId, hierarchyId) => {
   try {
     const response = await api.get(`/api/regions/${regionId}/subregions`, { params: { hierarchyId } });
     if (response.status === 204) {
-      // TODO return empty array instead of null, Issue #189
-      return null;
+      return [];
     }
     return response.data;
   } catch (error) {

frontend/src/components/RegionMap.jsx

@@ -64,9 +64,6 @@ function MapComponent() {
       // If region has subregions, fetch the subregions
       if (selectedRegion.hasSubregions) {
         const subregions = await fetchSubregions(selectedRegion.id, selectedHierarchy.hierarchyId);
-        if (subregions === null) {
-          return [];
-        }
         return subregions;
       }
       // If region does not have subregions, fetch the siblings