Description:
------------
Optional patterns, if not matched, seem to return NULL instead of empty strings in certain situations.

Test script:
---------------
var_dump([preg_match("!([a-z]+)://([^@]+@)?([a-z]+)!", "http://domain", $matches), $matches]);

Expected result:
----------------
array(2) {
  [0]=>
  int(1)
  [1]=>
  array(4) {
    [0]=>
    string(13) "http://domain"
    [1]=>
    string(4) "http"
    [2]=>
    string(0) ""
    [3]=>
    string(6) "domain"
  }
}


Actual result:
--------------
array(2) {
  [0]=>
  int(1)
  [1]=>
  array(4) {
    [0]=>
    string(13) "http://domain"
    [1]=>
    string(4) "http"
    [2]=>
    NULL
    [3]=>
    string(6) "domain"
  }
}