Use phpstan return logic to infer return type from argument

[ildyria]

As migrating to php 8.2, I had to change utf8_encode (soon deprecated) to mb_convert_encoding.
Unfortunately this function when given a string as input returns a string, and when given an array returns an array.
This fix aims to leverage on phpdoc @return logic to infer the correct return type.

[ildyria]

IDK why this is failing, I don't think the file I modified has impact on this. :|