- [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)
```

return isSupportResponseAttachment(version);
    }
    public static boolean isSupportResponseAttachment(String version) {
        if (StringUtils.isEmpty(version)) {
            return false;
        }
        
        
        int iVersion = getIntVersion(version);
        if (iVersion >= 2001000 && iVersion <= 2060200) {
            return false;
        }
        return iVersion >= LOWEST_VERSION_FOR_RESPONSE_ATTACHMENT;
    }
    public static int getIntVersion(String version) {
        Integer v = VERSION2INT.get(version);
        if (v == null) {
            v = parseInt(version);
            
            if (version.split("\\.").length == 3) {
                v = v * 100;
            }
            VERSION2INT.put(version, v);
        }
        return v;
    }
    private static int parseInt(String version) {
        int v = 0;
        String[] vArr = version.split("\\.");
        int len = vArr.length;