[5.2] Fix PDO connection on HHVM with the default "class without name" case

[vlakoff]

HHVM doesn't support using an empty array to omit the 3rd parameter "constructor argument".

Refs #14052 and #14426.

[vlakoff]

A simpler alternative could be by using:

if ($fetchMode === PDO::FETCH_CLASS && ! isset($fetchArgument)) {
    $fetchMode = PDO::FETCH_OBJ;
}

I was intending to use it in the first place, but changing $fetchMode sounded a bit risky to me.