allow openstack invetnory to work cacheless

fixes #45431

changelogs/fragments/openstack_inventory_fix.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - allow using openstack inventory plugin w/o a cache

lib/ansible/plugins/inventory/openstack.py

@@ -196,6 +196,7 @@ class InventoryModule(BaseInventoryPlugin, Constructable, Cacheable):
             source_data = cloud_inventory.list_hosts(
                 expand=expand_hostvars, fail_on_cloud_config=fail_on_errors)
 
+            if self.cache is not None:
                 self.cache.set(cache_key, source_data)
 
         self._populate_from_source(source_data)