Avoid empty print during validation workflow execution

Workflow check_pre_deployment_validations, sends multiple messages,
as part of the sub workflow execution. If the workflow is success,
there are no messages sent and it will be empty string. Avoid
printing empty strings.

Change-Id: I3f14a3bc97fa1e51230e1f6b1de4ed4fb8205842

tripleoclient/workflows/validations.py

@@ -27,7 +27,7 @@ def check_predeployment_validations(clients, **workflow_input):
     warnings = []
     with tripleoclients.messaging_websocket(queue_name) as ws:
         for payload in base.wait_for_messages(workflow_client, ws, execution):
-            if 'message' in payload:
+            if payload.get('message'):
                 print(payload['message'])
             if 'errors' in payload:
                 errors += payload['errors']