How do you figure?

A deep dive into figure accessibility from Scott O’Hara:

A figcaption is meant to provide a caption or summary to a figure, relating it back to the document the figure is contained within, or conveying additional information that may not be directly apparent from reviewing the figure itself.

If an image is given an empty alt, then the figcaption is in effect describing nothing. And that doesn’t make much sense, does it?