refactor(data/string/basic): simplify proof of `to_list_nonempty` (#6117)
refactor(data/string/basic): simplify proof of `to_list_nonempty` (#6117)

Co-authors: `lean-gptf`, Stanislas Polu

This was found by `formal-lean-wm-to-tt-m1-m2-v4-c4` when we evaluated it on theorems added to `mathlib` after we last extracted training data.



leanprover-community/mathlib,leanprover-community/mathlib