修正了文件不存在是sdk的NPE问题

mall-ebtp-cloud-attachment-sdk/src/main/java/com/chinaunicom/ebtp/mall/cloud/attachment/sdk/model/AttachmentDetail.java

@@ -24,4 +24,12 @@ public class AttachmentDetail extends Hashtable<String, List<AttachmentEntity>>
 		put(bid, list);
 	}
 
+	@Override
+	public synchronized List<AttachmentEntity> get(Object key) {
+		if (contains(key)) {
+			super.get(key);
+		}
+		return new ArrayList<>();
+	}
+
 }