Fix empty-category nav and route behavior

Use category-state-driven queries for nav and category listing routes, and add regression tests for empty but valid categories.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/blog/tests/test_views.py

@@ -233,3 +233,14 @@ def test_article_index_category_route_supports_tag_filter(client, home_page):
     assert resp.status_code == 200
     assert "Keep Me" in html
     assert "Drop Me" not in html
+
+
+@pytest.mark.django_db
+def test_article_index_category_route_allows_empty_existing_category(client, home_page):
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    Category.objects.create(name="Opinion", slug="opinion")
+
+    resp = client.get("/articles/category/opinion/")
+    assert resp.status_code == 200
+    assert "No articles found." in resp.content.decode()