- [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)
```

if (codeSource == null) {
                logger.info("No codeSource for class " + cls.getName() + " when getVersion, use default version " + defaultVersion);
                return defaultVersion;
            } 
            
            String file = codeSource.getLocation().getFile();
            if (!StringUtils.isEmpty(file) && file.endsWith(".jar")) {
                version = getFromFile(file);
            }
            
            
            return StringUtils.isEmpty(version) ? defaultVersion : version;
        } catch (Throwable e) {
            
            logger.error("return default version, ignore exception " + e.getMessage(), e);
            return defaultVersion;
        }
    }
    /**
     * get version from file: path/to/group-module-x.y.z.jar, returns x.y.z
     */
    private static String getFromFile(String file) {
        
        file = file.substring(0, file.length() - 4);
        
        
        int i = file.lastIndexOf('/');
        if (i >= 0) {
            file = file.substring(i + 1);
        }