fix: get_items without ids

Passing empty list to `search` has different semantics than `None`.

Author: gadomski

pystac_client/client.py

@@ -451,7 +451,7 @@ def get_items(self, *ids: str, recursive: bool = False) -> Iterator["Item_Type"]
                 catalog.
         """
         if self.conforms_to(ConformanceClasses.ITEM_SEARCH):
-            search = self.search(ids=ids)
+            search = self.search(ids=ids or None)
             yield from search.items()
         else:
             self._warn_about_fallback("ITEM_SEARCH")