fix: wrong mb_encoding_aliases() handling for PHP 8.0+

[uzulla]

The judgement that an empty array return from mb_encoding_aliases indicates an invalid encoding name was incorrect.

sample.

# php 8.0+

## get all list
php > var_dump(mb_list_encodings());
<snip>
  [61]=>
  string(2) "HZ"
<snip>
}

## HZ does not have aliases, but that's not an error.
php > var_dump(mb_encoding_aliases("HZ"));
array(0) {
}

# php 7.4 (same as 8.0+)
php > var_dump(mb_encoding_aliases("HZ"));
array(0) {
}

[oscarotero]

thank you!