avoids prompting for vars during syntax check

fixes #13319

lib/ansible/executor/playbook_executor.py

@@ -111,6 +111,9 @@ class PlaybookExecutor:
                             if vname not in play.vars:
                                 if self._tqm:
                                     self._tqm.send_callback('v2_playbook_on_vars_prompt', vname, private, prompt, encrypt, confirm, salt_size, salt, default)
+                                if self._options.syntax:
+                                    play.vars[vname] = default
+                                else:
                                     play.vars[vname] = self._do_var_prompt(vname, private, prompt, encrypt, confirm, salt_size, salt, default)
 
                     # Create a temporary copy of the play here, so we can run post_validate