Description:
------------
mb_detect_encoding with ommited encodings (= null as argument => fallback to default detect order) behaves different on 8.1.0beta2 and fails to detect UTF-8 in some cases with out-of-the-box settings.

I noticed this via the `ApplicationTest::testRenderExceptionWithDoubleWidthCharacters` test from Symfony in https://github.com/symfony/symfony/issues/41552 that broke because it converted UTF-8 to UTF-8 again.

Test script:
---------------
<?php

var_dump(mb_detect_order());
var_dump(mb_detect_encoding('コマンドの実行中にエラーが発生しました。', null, true));
var_dump(mb_detect_encoding('コマンドの実行中にエラーが発生しました', null, true));

Expected result:
----------------
array(2) {
  [0]=>
  string(5) "ASCII"
  [1]=>
  string(5) "UTF-8"
}
string(5) "UTF-8"
string(5) "UTF-8"

Actual result:
--------------
array(2) {
  [0]=>
  string(5) "ASCII"
  [1]=>
  string(5) "UTF-8"
}
string(5) "ASCII"
string(5) "UTF-8"