fix(bundler): reject falsy non-list 'catalogs' too (is None, not falsy)

Address review: the isinstance guard sat after `if not catalogs: return`,
so falsy non-list values (`catalogs: false`, `0`, `''`, `{}`) hit the
early return and were silently accepted instead of raising the promised
BundlerError. Only an absent/None value means "nothing to merge".

Change the early return to `if catalogs is None`, mirroring the sibling
reader (commands_impl/catalog_config._read). An empty list stays valid
(the merge loop is a no-op). Add parametrized tests for the falsy
non-list cases and for the absent/empty-list no-op.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jawwad-ali

src/specify_cli/bundler/models/catalog.py

@@ -251,14 +251,18 @@ def _merge_config(by_id: dict[str, CatalogSource], config_path: Path, scope: Sco
         return
     data = load_yaml(config_path)
     catalogs = data.get("catalogs") if isinstance(data, dict) else None
-    if not catalogs:
+    if catalogs is None:
         return
     if not isinstance(catalogs, list):
-        # A non-empty scalar (``catalogs: 5``) passes the falsy check above and
-        # would raise a raw ``TypeError: 'int' object is not iterable`` from the
-        # loop below. Report the same actionable BundlerError the sibling reader
-        # of this file raises (commands_impl/catalog_config.py) so both readers
-        # of bundle-catalogs.yml agree.
+        # Treat only an absent/``None`` ``catalogs`` as "nothing to merge"; any
+        # other non-list value (``catalogs: 5``, ``false``, ``0``, ``''``,
+        # ``{}``) is a malformed config and must raise, not be silently skipped
+        # by a falsy check. Otherwise a truthy scalar would raise a raw
+        # ``TypeError: 'int' object is not iterable`` from the loop below, while
+        # falsy non-lists would be swallowed. Report the same actionable
+        # BundlerError the sibling reader of this file raises
+        # (commands_impl/catalog_config.py) so both readers of
+        # bundle-catalogs.yml agree. An empty list stays valid (loop is a no-op).
         raise BundlerError(
             f"Malformed catalog config at {config_path}: 'catalogs' must be a "
             f"list, got {type(catalogs).__name__}."

tests/contract/test_catalog_schema.py

@@ -55,6 +55,30 @@ def test_non_list_catalogs_raises_actionable_error(tmp_path: Path):
         load_source_stack(tmp_path)
 
 
+@pytest.mark.parametrize("value", ["false", "0", "''", "{}"])
+def test_falsy_non_list_catalogs_still_raises(tmp_path: Path, value: str):
+    """A *falsy* non-list ``catalogs:`` value (false/0/''/{}) must also raise —
+    only an absent/``None`` value means "nothing to merge". A plain falsy check
+    would silently swallow these, diverging from the sibling reader."""
+    make_project(tmp_path)
+    (tmp_path / ".specify" / "bundle-catalogs.yml").write_text(
+        f"catalogs: {value}\n", encoding="utf-8"
+    )
+    with pytest.raises(BundlerError, match="must be a list"):
+        load_source_stack(tmp_path)
+
+
+@pytest.mark.parametrize("body", ["catalogs:\n", "catalogs: []\n"])
+def test_absent_or_empty_catalogs_is_noop(tmp_path: Path, body: str):
+    """An absent (``None``) or empty-list ``catalogs:`` is valid: it contributes
+    no project sources and falls back to the built-in default stack."""
+    make_project(tmp_path)
+    (tmp_path / ".specify" / "bundle-catalogs.yml").write_text(body, encoding="utf-8")
+    # Does not raise; still yields the built-in defaults.
+    sources = load_source_stack(tmp_path)
+    assert len(sources) > 0
+
+
 def test_project_config_overrides_same_id(tmp_path: Path):
     make_project(tmp_path)
     config = {