- [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)
```

String error = "Duplicate class " + path + " in " + files.size() + " jar " + files;
                if (failOnError) {
                    throw new IllegalStateException(error);
                } else {
                    logger.error(error);
                }
            }
        } catch (Throwable e) {
            logger.error(e.getMessage(), e);
        }
    }
    /**
     * search resources in caller's classloader
     */
    private static Set<String> getResources(String path) throws IOException {
        Enumeration<URL> urls = ClassHelper.getCallerClassLoader(Version.class).getResources(path);
        Set<String> files = new HashSet<String>();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            if (url != null) {
                String file = url.getFile();
                if (file != null && file.length() > 0) {
                    files.add(file);
                }
            }
        }
        return files;
    }
}