- [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)
```

import java.io.IOException;
import java.net.URL;
import java.security.CodeSource;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/**
 * Version
 */
public final class Version {
    private static final Logger logger = LoggerFactory.getLogger(Version.class);
    
    private static final Pattern PREFIX_DIGITS_PATTERN = Pattern.compile("^([0-9]*).*");
    
    public static final String DEFAULT_DUBBO_PROTOCOL_VERSION = "2.0.2";
    
    private static final String VERSION = getVersion(Version.class, "");
    /**
     * For protocol compatibility purpose.
     * Because {@link #isSupportResponseAttachment} is checked for every call, int compare expect to has higher
     * performance than string.
     */
    private static final int LOWEST_VERSION_FOR_RESPONSE_ATTACHMENT = 2000200; 
    private static final Map<String, Integer> VERSION2INT = new HashMap<String, Integer>();
    static {