Fix silent per-char iteration when DoFn returns str/bytes/dict - #38429

Failed to save: Unable to reserve cache with key tox-Linux-py3.10 3.10 -875f9432d967c2c45e963901f0a53fa44d67f03169d6d77a01d4740e680da9a9-b882e9797064ca3ef48de97ef298d95e3b864b250d94c1bc5d7f0c4afba7153a. More details: cache write denied: token has no writable scopes

Cache reservation failed: cache write denied: token has no writable scopes

Cache reservation failed: cache write denied: token has no writable scopes

Cache reservation failed: cache write denied: token has no writable scopes

Cache reservation failed: cache write denied: token has no writable scopes

Cache reservation failed: cache write denied: token has no writable scopes