- [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)
```

for (int i = 0; i < len; i++) {
            v += Integer.parseInt(getPrefixDigits(vArr[i])) * Math.pow(10, (len - i - 1) * 2);
        }
        return v;
    }
    /**
     * get prefix digits from given version string
     */
    private static String getPrefixDigits(String v) {
        Matcher matcher = PREFIX_DIGITS_PATTERN.matcher(v);
        if (matcher.find()) {
            return matcher.group(1);
        }
        return "";
    }
    public static String getVersion(Class<?> cls, String defaultVersion) {
        try {
            
            String version = cls.getPackage().getImplementationVersion();
            if (!StringUtils.isEmpty(version)) {
                return version;
            }
            
            version = cls.getPackage().getSpecificationVersion();
            if (!StringUtils.isEmpty(version)) {
                return version;
            }
            
            
            CodeSource codeSource = cls.getProtectionDomain().getCodeSource();