(v)sprintf() can return false on PHP7

[staabm]

we are missing sprintf/vsprintf can return false on php7

https://3v4l.org/aEqBC

[staabm]

hmm I wonder how annoying this change is in practice, as can be seen already at the phpstan own codebase (example), that this triggers lots of warnings.

@ondrejmirtes wdyt about turning this into a benevolent (string|false) on php7?

[ondrejmirtes]

The fact that there's no bug report for this means that this false rarely happens in practice and gets rarely handled which means I don't want to add it to the return type on PHP 7.x.