should variables apply when defaultMessage is used?

Hi,
Not sure if it is a bug, but with my usage of extract-intl, I had a case, when the locale message was actually an empty string, so we passed [this null check](https://github.com/alibaba/react-intl-universal/blob/master/src/index.js#L49) variables had no effect, and the defaultMessage is shown in the end with {variable} as in "locale.json".

I don't know what is a right thing to do, but it seems to me that the desired result would be to have variables inserted into defaultMessage as well.

Thanks!

Start by reading the null check at src/index.js line 49 and trace how extract-intl handles an empty locale message. Reproduce the case with a defaultMessage containing {variable}, then verify that the chosen behavior is covered and that variables are handled consistently when the defaultMessage is shown.

Written by the indexing model from the issue text.