[SC-18090] Treat a missing partial index as empty when merging Quarto indexes

.github/scripts/merge_quarto_indexes.py

@@ -27,7 +27,10 @@ def merge_items(
 
 def merge_file(base_path: Path, partial_path: Path, key: str) -> None:
     base = json.loads(base_path.read_text())
-    partial = json.loads(partial_path.read_text())
+    # Quarto only writes listings.json when a rendered page declares a listing,
+    # so a targeted render of pages that carry none produces no partial index.
+    # That is a normal outcome, not a failure: nothing changed, nothing to merge.
+    partial = json.loads(partial_path.read_text()) if partial_path.exists() else []
     if not isinstance(base, list) or not isinstance(partial, list):
         raise ValueError("Quarto indexes must contain JSON arrays")
     partial_path.write_text(

.github/scripts/test_merge_quarto_indexes.py

@@ -40,6 +40,20 @@ def test_merge_file_updates_partial_path(self):
                 ],
             )
 
+    def test_absent_partial_leaves_base_as_the_merged_index(self):
+        """A targeted render of pages with no listing writes no partial index."""
+        with tempfile.TemporaryDirectory() as directory:
+            base_path = Path(directory) / "base.json"
+            partial_path = Path(directory) / "partial.json"
+            base_path.write_text(json.dumps([{"listing": "/old", "items": ["a"]}]))
+
+            merge_file(base_path, partial_path, "listing")
+
+            self.assertEqual(
+                json.loads(partial_path.read_text()),
+                [{"listing": "/old", "items": ["a"]}],
+            )
+
     def test_missing_key_is_rejected(self):
         with self.assertRaises(ValueError):
             merge_items([], [{"text": "missing object id"}], "objectID")