Add test, update changelog

Author: jsignell

CHANGELOG.md

@@ -18,6 +18,7 @@
 - Expand support for previous extension schema URIs ([#1091](https://github.com/stac-utils/pystac/pull/1091))
 - Use `pyproject.toml` instead of `setup.py` ([#1100](https://github.com/stac-utils/pystac/pull/1100))
 - `DefaultStacIO` now raises an error if it tries to write to a non-local url ([#1107](https://github.com/stac-utils/pystac/pull/1107))
+- Allow instantiation of pystac objects even with `"stac_extensions": null` ([#1109](https://github.com/stac-utils/pystac/pull/1109))
 
 ### Deprecated
 

tests/serialization/test_migrate.py

@@ -99,3 +99,13 @@ def test_should_raise_exception_when_passing_invalid_extension_object(
             match=r"^Item Assets extension does not apply to type 'object'$",
         ):
             ItemAssetsExtension.ext(object())  # type: ignore
+
+
+def test_migrate_works_even_if_stac_extensions_is_null(
+    test_case_1_catalog: pystac.Catalog,
+) -> None:
+    collection = list(test_case_1_catalog.get_all_collections())[0]
+    collection_dict = collection.to_dict()
+    collection_dict["stac_extensions"] = None
+
+    pystac.Collection.from_dict(collection_dict, migrate=True)