avoid posting related topics on empty issues, and close them

[jba]

https://go.dev/issue/69795 was posted with an empty issue body. Gaby should close such issues.
Instead it posted a "related issues" comment (which included other empty issues—good job there).

[hyangah]

another approach (from @ianthehat) is to let LLM classify as part of gaby's regular labeling task, and don't trigger related topics / summary posting.