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.

[hyangah]

Another thought while reviewing CL 645875:

We are doing markdown parsing. If the contents have only headings and the contents after removing headings are little, it's likely a spam or empty.

Another floating idea:

If the similarity search finds only issues with "empty" or "invalid" label, we can assume it is similarly empty or invalid. (depending on which LLM call is cheaper)

[gopherbot]

Change https://go.dev/cl/636875 mentions this issue: internal/labels: check for empty issue