- [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)
```

i = file.indexOf("-");
        if (i >= 0) {
            file = file.substring(i + 1);
        }
        
        
        while (file.length() > 0 && !Character.isDigit(file.charAt(0))) {
            i = file.indexOf("-");
            if (i >= 0) {
                file = file.substring(i + 1);
            } else {
                break;
            }
        }
        return file;
    }
    public static void checkDuplicate(Class<?> cls, boolean failOnError) {
        checkDuplicate(cls.getName().replace('.', '/') + ".class", failOnError);
    }
    public static void checkDuplicate(Class<?> cls) {
        checkDuplicate(cls, false);
    }
    public static void checkDuplicate(String path, boolean failOnError) {
        try {
            
            Set<String> files = getResources(path);
            
            if (files.size() > 1) {