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://..."/>
```
@SuppressWarnings("unchecked")
    private Invoker<T> getInvoker(String mockService) {
        Invoker<T> invoker = (Invoker<T>) mocks.get(mockService);
        if (invoker != null) {
            return invoker;
        }
        Class<T> serviceType = (Class<T>) ReflectUtils.forName(url.getServiceInterface());
        T mockObject = (T) getMockObject(mockService, serviceType);
        invoker = proxyFactory.getInvoker(mockObject, serviceType, url);
        if (mocks.size() < 10000) {
            mocks.put(mockService, invoker);
        }
        return invoker;
    }
    @SuppressWarnings("unchecked")
    public static Object getMockObject(String mockService, Class serviceType) {
        if (ConfigUtils.isDefault(mockService)) {
            mockService = serviceType.getName() + "Mock";
        }
        Class<?> mockClass = ReflectUtils.forName(mockService);
        if (!serviceType.isAssignableFrom(mockClass)) {
            throw new IllegalStateException("The mock class " + mockClass.getName() +
                    " not implement interface " + serviceType.getName());
        }
        try {
            return mockClass.newInstance();
        } catch (InstantiationException e) {
            throw new IllegalStateException("No default constructor from mock class " + mockClass.getName(), e);
        } catch (IllegalAccessException e) {
            throw new IllegalStateException(e);