return empty host list when pattern is empty

fixes #37894

(cherry picked from commit 2e852fcd6d)

changelogs/fragments/empty_host_pattern.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - return empty list if host pattern is empty https://github.com/ansible/ansible/pull/37931

lib/ansible/inventory/manager.py

@@ -330,17 +330,20 @@ 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 += ":%s" % to_native(self._subset)
+                pattern_hash += u":%s" % to_text(self._subset, errors='surrogate_or_strict')
 
             if not ignore_restrictions and self._restriction:
-            pattern_hash += ":%s" % to_native(self._restriction)
+                pattern_hash += u":%s" % to_text(self._restriction, errors='surrogate_or_strict')
 
             if pattern_hash not in self._hosts_patterns_cache: