@b4n the only one that I would question is the one that @eht16 had questioned, so I was waiting until that was decided. :-P
"Data contains NULs" is referring to the result of the conversion, but the user doesn't know that, they only have the input file and it may not contain NULs, eg it might have an overlong encoding of a nul.
So "Result of conversion to UTF-8 buffer contains NULs" might be better.