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 (value.length() == 0 || value.equalsIgnoreCase("false")) {
            
            result = this.invoker.invoke(invocation);
        } else if (value.startsWith("force")) {
            if (logger.isWarnEnabled()) {
                logger.warn("force-mock: " + invocation.getMethodName() + " force-mock enabled , url : " + directory.getUrl());
            }
            
            result = doMockInvoke(invocation, null);
        } else {
            
            try {
                result = this.invoker.invoke(invocation);
            } catch (RpcException e) {
                if (e.isBiz()) {
                    throw e;
                }
                
                if (logger.isWarnEnabled()) {
                    logger.warn("fail-mock: " + invocation.getMethodName() + " fail-mock enabled , url : " + directory.getUrl(), e);
                }
                result = doMockInvoke(invocation, e);
            }
        }
        return result;
    }
    @SuppressWarnings({"unchecked", "rawtypes"})
    private Result doMockInvoke(Invocation invocation, RpcException e) {
        Result result = null;
        Invoker<T> minvoker;