Allow *bin2hex and *bin2base64 functions to keep non-empty-string type
Allow *bin2hex and *bin2base64 functions to keep non-empty-string type

Those functions should not return a string when they receive a
non-empty-string in input.

The following example is expected to work:
```php
<?php

/**
 * @param non-empty-string $i
 */
function takesNonEmptyString(string $i): void {
    echo $i;
}

takesNonEmptyString(bin2hex("a"));
takesNonEmptyString(base64_encode("a"));
```

vimeo/psalm,vimeo/psalm