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://..."/>
```
}
    }
    /**
     * Normalize mock string:
     *
     * <ol>
     * <li>return => return null</li>
     * <li>fail => default</li>
     * <li>force => default</li>
     * <li>fail:throw/return foo => throw/return foo</li>
     * <li>force:throw/return foo => throw/return foo</li>
     * </ol>
     *
     * @param mock mock string
     * @return normalized mock string
     */
    public static String normalizeMock(String mock) {
        if (mock == null) {
            return mock;
        }
        mock = mock.trim();
        if (mock.length() == 0) {
            return mock;
        }
        if (Constants.RETURN_KEY.equalsIgnoreCase(mock)) {
            return Constants.RETURN_PREFIX + "null";
        }
        if (ConfigUtils.isDefault(mock) || "fail".equalsIgnoreCase(mock) || "force".equalsIgnoreCase(mock)) {
            return "default";
        }