Description:
------------
when executing pg_execute it modifies the null values in 
the params array to an empty string

Reproduce code:
---------------
$conn = pg_connect('dbname=test');

$param_list = array(null, );

var_dump($param_list);

pg_prepare($conn, 'test', 'INSERT INTO nullproblem2 VALUES ($1)');
pg_execute($conn, 'test', $param_list);

var_dump($param_list);


Expected result:
----------------
array(1) {
  [0]=>
  NULL
}
array(1) {
  [0]=>
  NULL
}


Actual result:
--------------
array(1) {
  [0]=>
  NULL
}
array(1) {
  [0]=>
  string(0) ""
}