NPE when package of interface is empty.

[ujjboy]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ 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.

2. Export service.

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)