infer non-empty-string on `strlen() == strlen()`

### Feature request

https://phpstan.org/r/5c29b9e7-4bf1-4064-8658-8dacdc9d46b3

no error

analog https://github.com/phpstan/phpstan/issues/13675 but for strings

### Did PHPStan help you today? Did it make you happy in any way?

_No response_