Cyberstorm API: return empty PackageVersion.website_url as None

The field works on the database level a bit differently than the other
fields in that it allows empty values to be stored as empty strings
rather than nulls.

Hide this implementation detail and keep the API consistent by
returning None if the field is in fact empty.

Refs TS-1980

django/thunderstore/api/cyberstorm/tests/test_package_details.py

@@ -194,5 +194,28 @@ def test_package_detail_view__returns_info(api_client: APIClient) -> None:
     assert actual["website_url"] == latest.website_url
 
 
+@pytest.mark.django_db
+def test_package_detail_view__serializes_url_correctly(api_client: APIClient) -> None:
+    l = PackageListingFactory(
+        package_version_kwargs={
+            "website_url": "https://thunderstore.io/",
+        },
+    )
+
+    url = f"/api/cyberstorm/package/{l.community.identifier}/{l.package.namespace}/{l.package.name}/"
+    response = api_client.get(url)
+    actual = response.json()
+
+    assert actual["website_url"] == "https://thunderstore.io/"
+
+    l.package.latest.website_url = ""
+    l.package.latest.save(update_fields=("website_url",))
+
+    response = api_client.get(url)
+    actual = response.json()
+
+    assert actual["website_url"] == None
+
+
 def _date_to_z(value: datetime) -> str:
     return value.strftime("%Y-%m-%dT%H:%M:%S.%fZ")

django/thunderstore/api/cyberstorm/views/package_detail.py

@@ -37,6 +37,18 @@ class TeamSerializer(serializers.Serializer):
     members = CyberstormTeamMemberSerializer(many=True)
 
 
+class EmptyStringAsNoneField(serializers.Field):
+    """
+    Serialize empty string to None and deserialize vice versa.
+    """
+
+    def to_representation(self, value):
+        return None if value == "" else value
+
+    def to_internal_value(self, data):
+        return "" if data is None else data
+
+
 class ResponseSerializer(serializers.Serializer):
     """
     Data shown on package detail view.
@@ -69,7 +81,7 @@ class ResponseSerializer(serializers.Serializer):
     rating_count = serializers.IntegerField(min_value=0)
     size = serializers.IntegerField(min_value=0, source="package.latest.file_size")
     team = TeamSerializer(source="package.owner")
-    website_url = serializers.CharField(source="package.latest.website_url")
+    website_url = EmptyStringAsNoneField(source="package.latest.website_url")
 
     def get_has_changelog(self, listing: PackageListing) -> bool:
         changelog = listing.package.latest.changelog