Set collection to None in item constructor when there is no collection (#1400)

* fix: set collection to None in item constructor

* chore: update changelog

Author: gadomski

CHANGELOG.md

@@ -18,6 +18,7 @@
 ### Fixed
 
 - Make `get_all_collections` properly recursive ([#1361](https://github.com/stac-utils/pystac/pull/1361))
+- Set `Item::collection` to `None` when there is no collection ([#1400](https://github.com/stac-utils/pystac/pull/1400))
 
 ### Removed
 

pystac/item.py

@@ -158,7 +158,9 @@ def __init__(
             self.set_self_href(href)
 
         self.collection_id: str | None = None
-        if collection is not None:
+        if collection is None:
+            self.collection = None
+        else:
             if isinstance(collection, Collection):
                 self.set_collection(collection)
             else:

tests/test_item.py

@@ -692,3 +692,8 @@ def test_copy_with_unresolveable_root(item: Item) -> None:
         )
     )
     copy.deepcopy(item)
+
+
+def test_no_collection(item: Item) -> None:
+    # https://github.com/stac-utils/stac-api-validator/issues/527
+    assert item.collection is None