Mediawiki user can enable preference setting "Prompt me when entering a blank edit summary" (Editing/Editor section). Having this setting on, and leaving the summary field empty while doing e.g. edit in the wikitext editor results in the warning prompt.
Entity Schema editor should behave in similar manner when the setting is on.
[ ] When the user has enabled "Prompt me when entering a blank edit summary" in their preferences and is saving the Entity Schema change with the black edit summary, there should be a warning shown.