Correctly initialize combined_cache for hosts if setup is skipped

Fixes #7364

lib/ansible/runner/__init__.py

@@ -566,7 +566,7 @@ class Runner(object):
 
         # merge the VARS and SETUP caches for this host
         combined_cache = self.setup_cache.copy()
-        combined_cache.get(host, {}).update(self.vars_cache.get(host, {}))
+        combined_cache.setdefault(host, {}).update(self.vars_cache.get(host, {}))
         hostvars = HostVars(combined_cache, self.inventory, vault_password=self.vault_pass)
 
         # use combined_cache and host_variables to template the module_vars