List of steps to reproduce (step by step, including full links if applicable):

• On a wiki where you have the flow-delete right, create a test topic.
• Start the deletion dialog for that topic
• Try submitting without a summary

What happens?:
The "delete" button is grayed out, and pressing "enter" in the text box does nothing.

What should have happened instead?:
The topic should be deleted without a summary, same as when deleting anything else.

There are valid reasons to delete something without a summary. For instance, when deleting mass creations by an LTA, many admins will delete without summary as part of denying recognition. That's the use case in which I encountered this issue here. There may be some who consider no-summary deletions suboptimal in all cases, but that doesn't matter: Software shouldn't be dictating local practices regarding deletion—or, even if it should, it should be consistent across different types of deletion. I see no reason that I should be able to delete an article without a summary, but not delete a topic on its talk page without one.