Update the mbstring.http_input and mbstring.http_output configuration check, from being the deprecated PHP 5.6.x "PASS, or blank" to now be "UTF-8, or blank".  PHP documentation specifies this configuration should be left blank, in order to inherit default_charset, which phpBB requires to be configured as UTF-8.

For a phpBB customer where mbstring.http_input and mbstring.http_output is set to UTF-8 rather than being left blank, and is a shared hosting configuration that the customer isn't permitted to change, the phpBB ACP warning that "your http_input/http_output configuration is invalid" isn't helpful.

Their setting is UTF-8, which is the encoding http_input/http_output would be using if it had been left blank and allowed to inherit default_charset.  Our "warning" isn't advising the user of a misconfiguration which is causing a phpBB functionality problem.  phpBB is getting exactly the UTF-8 encoding it was intending to get; same as when it was checking whether the setting was left blank, too.

Also update the default_charset test to permit "UTF-8, or blank".  This had been correctly changed to effectively test "UTF-8, or NULL" during PHPBB3-16698, based on code inspection of /vendor/bantu/ini-get-wrapper/src/IniGetWrapper.php which confirmed IniGetWrapper::getString() will return NULL for any setting which does not exist.

However, testing on some PHP platforms shows PHP itself returning an empty string for a parameter which isn't defined in the PHP.INI, and so we receive an empty string instead of NULL.  So the test was updated so that we effectively check for "UTF-8, or blank, or NULL", all of which result in UTF-8 being used.

PHPBB3-16708

Fixed short array syntax, moved testing logic into member helper function.

PHPBB3-16708