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 directory.list() returns more than one mock invoker, only one of them will be used.
     *
     * @param invocation
     * @return
     */
    private List<Invoker<T>> selectMockInvoker(Invocation invocation) {
        List<Invoker<T>> invokers = null;
        //
        if (invocation instanceof RpcInvocation) {
            //No
            ((RpcInvocation) invocation).setAttachment(Constants.INVOCATION_NEED_MOCK, Boolean.TRUE.toString());
            //di
            try {
                invokers = directory.list(invocation);
            } catch (RpcException e) {
                if (logger.isInfoEnabled()) {
                    logger.info("Exception when try to invoke mock. Get mock invokers error for service:"
                            + directory.getUrl().getServiceInterface() + ", method:" + invocation.getMethodName()
                            + ", will contruct a new mock with 'new MockInvoker()'.", e);
                }
            }
        }
        return invokers;
    }
    @Override
    public String toString() {
        return "invoker :" + this.invoker + ",directory: " + this.directory;
    }
}