org.apache.dubbo.rpc.support.MockInvoker#getInterface should not return null, otherwise there's no way to specify URL with mock protocol in reference bean.

```xml
<dubbo:reference id="demoService" check="false" interface="o.DemoService" url="mock://..."/>
```
import org.apache.dubbo.rpc.support.MockInvoker;
import java.util.List;
public class MockClusterInvoker<T> implements Invoker<T> {
    private static final Logger logger = LoggerFactory.getLogger(MockClusterInvoker.class);
    private final Directory<T> directory;
    private final Invoker<T> invoker;
    public MockClusterInvoker(Directory<T> directory, Invoker<T> invoker) {
        this.directory = directory;
        this.invoker = invoker;
    }
    @Override
    public URL getUrl() {
        return directory.getUrl();
    }
    @Override
    public boolean isAvailable() {
        return directory.isAvailable();
    }
    @Override
    public void destroy() {
        this.invoker.destroy();
    }
    @Override
    public Class<T> getInterface() {
        return directory.getInterface();
    }
    @Override
    public Result invoke(Invocation invocation) throws RpcException {
        Result result = null;
        String value = directory.getUrl().getMethodParameter(invocation.getMethodName(), Constants.MOCK_KEY, Boolean.FALSE.toString()).trim();