If you hover over the episode number in the episode list you will see the pencil icon, leading directly to the episode edit page. Probably the fastest way would be to open it in a separate tab, edit, save and close.
@blackcat2021 wrote: A "Bulk summary" option would be helpful for the shows with a lot of episodes that have summary empty.
The only way, now, is to browse episode by episode and add a summary. (or is it an easier way that I don't know?)
This is not something that would work for most shows. Dramas, comedies, every episode is different. I'd rather have a blank summary, than something generic that doesn't fit. I suppose it could work for game shows/reality shows/competition shows, but even those can have episode-specific summaries in the press releases.