Utility::ValidateUTF8(): move a string instead of copying a vector

[Al2Klimov]

less malloc() = more speed

Especially as JsonEncode() validates every single input string.

fixes #9506

[Al2Klimov]

Unit tests ftw!

[julianbrost]

The change is fine but there's more room for optimization. Typically, the function should do nothing as the string should usually only contain valid UTF-8. So in this case, no copy would be needed at all. But achieving this isn't really possible with the current signature, and would likely require touching all callers.

[yhabteab]

So in this case, no copy would be needed at all. But achieving this isn't really possible with the current signature, and would likely require touching all callers.

I'm currently trying to achieve this!