- [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)
```

Version.checkDuplicate(Version.class);
    }
    private Version() {
    }
    public static String getProtocolVersion() {
        return DEFAULT_DUBBO_PROTOCOL_VERSION;
    }
    public static String getVersion() {
        return VERSION;
    }
    /**
     * Check the framework release version number to decide if it's 2.7.0 or higher
     */
    public static boolean isRelease270OrHigher(String version) {
        if (StringUtils.isEmpty(version)) {
            return false;
        }
        if (getIntVersion(version) >= 2070000) {
            return true;
        }
        return false;
    }
    /**
     * Check the framework release version number to decide if it's 2.6.3 or higher
     *
     * Because response attachments feature is firstly introduced in 2.6.3
     * and moreover we have no other approach to know the framework's version, so we use
     * isSupportResponseAttachment to decide if it's v2.6.3.
     */
    public static boolean isRelease263OrHigher(String version) {