When hovering over a link to an article with an empty extract (as returned by the TextExtracts extension) or an extract that is tidied to '', I no longer see the generic preview.
Dwelling on the "Yorkshire Terrier" link (and many, many others) on https://en.wikipedia.beta.wmflabs.org/wiki/Dog yields the following preview:
Under the hood, PP is making an HTTP GET request to https://en.wikipedia.beta.wmflabs.org/api/rest_v1/page/summary/Yorkshire_Terrier, which has an extract of "(Blank.)".
This affects the next release branch (-wmf.5).
Overzealous – but still awesome!!1 – refactoring 💥 🦄