backwards compatible recursive check for None

Co-authored-by: Julia Signell <jsignell@gmail.com>

Author: mishaschwartz

pystac_client/client.py

@@ -457,7 +457,7 @@ def get_items(self, *ids: str, recursive: bool = True) -> Iterator["Item_Type"]:
                 catalog.
         """
         if self.conforms_to(ConformanceClasses.ITEM_SEARCH):
-            if recursive:
+            if recursive is not False:
                 search = self.search(ids=ids)
                 try:
                     yield from search.items()