Fix `last.ckpt` only being saved when another checkpoint has been created (#21244)

* fix implementation

* add testing

* smaller fix

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* changelog

* empty ci commit

* fix tests

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Justus Schock <justus.schock@posteo.de>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>
Co-authored-by: Deependu <deependujha21@gmail.com>