Allow nullable stac_extensions

[jsignell]

Description: I noticed recently while working on VEDA (https://staging-stac.delta-backend.com/) that if stac_extensions is set to null, with #1091 the collection can't be opened so as not to break people who might have have this mild invalidity in their catalogs.

NOTE: I none of these collections with stac_extensions set to null fail validation because by the time you have created a collection from a dict and then dumped it back to a dict null has been stripped out.

Example:

from pystac import Catalog

catalog = Catalog.from_file('https://staging-stac.delta-backend.com/')
failed = []
for c in catalog.get_collections():
    try:
        c.validate()
    except:
        failed.append(c)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 5
      3 catalog = Catalog.from_file('https://staging-stac.delta-backend.com/')
      4 failed = []
----> 5 for c in catalog.get_collections():
      6     try:
      7         c.validate()

File ~/pystac/pystac/stac_object.py:379, in STACObject.get_stac_objects(self, rel, typ, modify_links)
    377 link = links[i]
    378 if link.rel == rel:
--> 379     link.resolve_stac_object(root=self.get_root())
    380     if typ is None or isinstance(link.target, typ):
    381         yield cast(STACObject, link.target)

File ~/pystac/pystac/link.py:322, in Link.resolve_stac_object(self, root)
    319     if stac_io is None:
    320         stac_io = pystac.StacIO.default()
--> 322 obj = stac_io.read_stac_object(target_href, root=root)
    323 obj.set_self_href(target_href)
    324 if root is not None:

File ~/pystac/pystac/stac_io.py:236, in StacIO.read_stac_object(self, source, root, *args, **kwargs)
    216 """Read a STACObject from a JSON file at the given source.
    217 
    218 See :func:`StacIO.read_text <pystac.StacIO.read_text>` for usage of
   (...)
    233     contained in the file at the given uri.
    234 """
    235 d = self.read_json(source, *args, **kwargs)
--> 236 return self.stac_object_from_dict(
    237     d, href=source, root=root, preserve_dict=False
    238 )

File ~/pystac/pystac/stac_io.py:168, in StacIO.stac_object_from_dict(self, d, href, root, preserve_dict)
    163     merge_common_properties(
    164         d, json_href=href_str, collection_cache=collection_cache
    165     )
    167 info = identify_stac_object(d)
--> 168 d = migrate_to_latest(d, info)
    170 if info.object_type == pystac.STACObjectType.CATALOG:
    171     result = pystac.Catalog.from_dict(
    172         d, href=href_str, root=root, migrate=False, preserve_dict=preserve_dict
    173     )

File ~/pystac/pystac/serialization/migrate.py:198, in migrate_to_latest(json_dict, info)
    195         result["stac_extensions"] = []
    197 pystac.EXTENSION_HOOKS.migrate(result, version, info)
--> 198 for ext in result["stac_extensions"][:]:
    199     if ext in removed_extension_migrations:
    200         object_types, migration_fn = removed_extension_migrations[ext]

TypeError: 'NoneType' object is not subscriptable

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[gadomski]

Change seems fine, but this could use a unit test? Also needs CHANGELOG entry.