- [x] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.0

### Steps to reproduce this issue

1. Interface without package.

![image](https://user-images.githubusercontent.com/1424920/53320488-ca6e5880-3910-11e9-9493-e074ff1f91a7.png)

2. Export service.
```java
serviceConfig.setInterface(HelloService.class.getName());
serviceConfig.setRef(new HelloServiceImpl());
```

### Expected Result

Export service.

### Actual Result

Catch NPE when export service.

```
java.lang.NullPointerException
	at com.alibaba.dubbo.common.Version.getVersion(Version.java:127)
	at com.alibaba.dubbo.config.ServiceConfig.doExportUrlsFor1Protocol(ServiceConfig.java:440)
	at com.alibaba.dubbo.config.ServiceConfig.doExportUrls(ServiceConfig.java:358)
	at com.alibaba.dubbo.config.ServiceConfig.doExport(ServiceConfig.java:317)
```

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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
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package org.apache.dubbo.common;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.utils.ClassHelper;
import org.apache.dubbo.common.utils.StringUtils;