[ML] Set createTime if not specified in validate api too

Original commit: elastic/x-pack-elasticsearch@477fb0fc52

plugin/src/main/java/org/elasticsearch/xpack/ml/action/ValidateJobConfigAction.java

@@ -68,6 +68,9 @@ extends Action<ValidateJobConfigAction.Request, ValidateJobConfigAction.Response
             // When jobs are PUT their ID must be supplied in the URL - assume this will
             // be valid unless an invalid job ID is specified in the JSON to be validated
             job.setId(job.getId() != null ? job.getId() : "ok");
+            if (job.getCreateTime() == null) {
+                job.setCreateTime(new Date());
+            }
             return new Request(job.build());
         }