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://..."/>
```
if (mock.startsWith(Constants.FAIL_PREFIX)) {
            mock = mock.substring(Constants.FAIL_PREFIX.length()).trim();
        }
        if (mock.startsWith(Constants.FORCE_PREFIX)) {
            mock = mock.substring(Constants.FORCE_PREFIX.length()).trim();
        }
        if (mock.startsWith(Constants.RETURN_PREFIX) || mock.startsWith(Constants.THROW_PREFIX)) {
            mock = mock.replace('`', '"');
        }
        return mock;
    }
    @Override
    public URL getUrl() {
        return this.url;
    }
    @Override
    public boolean isAvailable() {
        return true;
    }
    @Override
    public void destroy() {
        
    }
    @Override
    public Class<T> getInterface() {
        
        return null;
    }
}