Split off from T60975#1822872. At https://www.mediawiki.org/w/index.php?title=Topic:Sq8d47itaqbchwqh&action=history , an empty topic summary was created on November 21, 2015. We shouldn't allow empty topic summaries to be saved (or silently not record them, like we do with null edits).

Easy to reproduce by choosing "Summarize" for a topic that doesn't yet have a summary, and saving an empty summary.