Description:
------------
The documentation is unclear on this aspect so I am presuming this is a bug:

Keys in INI files set to null (no quotes) are parsed as empty string "" instead of a true null.

If this is intended behavior then it should be documented as such.

Reproduce code:
---------------
-- test.ini --
[foo]
bar = null

-- test.php --
var_dump( parse_ini_file( "test.ini" ) );

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

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