Recent sprintf changes

[jack-worman]

Concerns about the changes in #8922:

The new return type is not always true. Here is a counter example:
var_dump(sprintf('%s', '', 'x')); // string(0) ""

And this also does not figure out this is a non-empty-string:
var_dump(sprintf('%s%s', '', 'x')); // string(0) "x"

[jack-worman]

Nevermind, I think I am wrong because it defaults back to just string when there are more than 2 args