Truly check for `! empty()` instead of falsey `''` when determining whether to add `LIMIT` clause to SQL in `wp_get_archives()`.

Props jjeaton for the initial patch.
Fixes #27834.


git-svn-id: https://develop.svn.wordpress.org/trunk@28560 602fd350-edb4-49c9-b593-d223f7449a82