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://..."/>
```
@Override
    public int getDefaultPort() {
        return 0;
    }
    @Override
    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        throw new UnsupportedOperationException();
    }
    @Override
    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        return new MockInvoker<T>(url);
    }
}