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://..."/>
```
List<Invoker<T>> mockInvokers = selectMockInvoker(invocation);
        if (CollectionUtils.isEmpty(mockInvokers)) {
            minvoker = (Invoker<T>) new MockInvoker(directory.getUrl());
        } else {
            minvoker = mockInvokers.get(0);
        }
        try {
            result = minvoker.invoke(invocation);
        } catch (RpcException me) {
            if (me.isBiz()) {
                result = new RpcResult(me.getCause());
            } else {
                throw new RpcException(me.getCode(), getMockExceptionMessage(e, me), me.getCause());
            }
        } catch (Throwable me) {
            throw new RpcException(getMockExceptionMessage(e, me), me.getCause());
        }
        return result;
    }
    private String getMockExceptionMessage(Throwable t, Throwable mt) {
        String msg = "mock error : " + mt.getMessage();
        if (t != null) {
            msg = msg + ", invoke error is :" + StringUtils.toString(t);
        }
        return msg;
    }
    /**
     * Return MockInvoker
     * Contract：
     * directory.list() will return a list of normal invokers if Constants.INVOCATION_NEED_MOCK is present in invocation, otherwise, a list of mock invokers will return.