return empty host list when pattern is empty

fixes #37894

lib/ansible/inventory/manager.py

@@ -330,12 +330,15 @@ class InventoryManager(object):
         or applied subsets
         """
 
+        hosts = []
+
         # Check if pattern already computed
         if isinstance(pattern, list):
             pattern_hash = u":".join(pattern)
         else:
             pattern_hash = pattern
 
+        if pattern_hash:
             if not ignore_limits and self._subset:
                 pattern_hash += u":%s" % to_text(self._subset, errors='surrogate_or_strict')