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.common.Constants;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.ExtensionLoader;
import org.apache.dubbo.common.utils.ConfigUtils;
import org.apache.dubbo.common.utils.PojoUtils;
import org.apache.dubbo.common.utils.ReflectUtils;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.common.utils.ArrayUtils;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.ProxyFactory;
import org.apache.dubbo.rpc.Result;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.RpcInvocation;
import org.apache.dubbo.rpc.RpcResult;
import com.alibaba.fastjson.JSON;
import java.lang.reflect.Constructor;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
final public class MockInvoker<T> implements Invoker<T> {
    private final static ProxyFactory proxyFactory = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
    private final static Map<String, Invoker<?>> mocks = new ConcurrentHashMap<String, Invoker<?>>();
    private final static Map<String, Throwable> throwables = new ConcurrentHashMap<String, Throwable>();
    private final URL url;
    public MockInvoker(URL url) {
        this.url = url;
    }
    public static Object parseMockValue(String mock) throws Exception {