fix: Allow null for Region.main_generation in serializer

Some regions (e.g. Hisui) have no main_generation, so the API returns
null for this field. Adding allow_null=True to the serializer field
so the generated OpenAPI spec correctly marks it as nullable.

Fixes #1397

Author: santichausis

openapi.yml

@@ -8607,7 +8607,6 @@ components:
           allOf:
           - $ref: '#/components/schemas/GenerationSummary'
           readOnly: true
-          nullable: true
         names:
           type: array
           items:
@@ -8639,6 +8638,7 @@ components:
       required:
       - id
       - locations
+      - main_generation
       - name
       - names
       - pokedexes

pokemon_v2/serializers.py

@@ -605,7 +605,7 @@ class RegionDetailSerializer(serializers.ModelSerializer):
     locations = LocationSummarySerializer(many=True, read_only=True, source="location")
     version_groups = serializers.SerializerMethodField("get_region_version_groups")
     pokedexes = PokedexSummarySerializer(many=True, read_only=True, source="pokedex")
-    main_generation = GenerationSummarySerializer(read_only=True, source="generation")
+    main_generation = GenerationSummarySerializer(read_only=True, source="generation", allow_null=True)
 
     class Meta:
         model = Region