Description:
------------
There is an unexpected (or undocumented) aspect of money_format.  When supplied with 0 or null as a value it returns the formatted version of those values.  When an empty string is supplied as the value, null is returned.  Null and '' should be treated the same way in this case.

This behavior may be understandable when a non-numeric string is submitted, but a notice should probably be emitted.  

Reproduce code:
---------------
<?php
ini_set('display_errors',true);
error_reporting(E_ALL & E_STRICT);

foreach(array("0"=>0,"''"=>"",'null'=>null,"'dummy'"=>'dummy',"'2'"=>'2') as $desc=>$value) {
  $mf = money_format("%(n",$value);
  echo "value: $desc\tformat:'".$mf."'\tis_null: ".(string)(int)is_null($mf)."\n";
}


Expected result:
----------------
value: 0	format:'0.00'	is_null: 0
value: ''	format:''	is_null: 1
value: null	format:'0.00'	is_null: 0
value: 'dummy'	format:''	is_null: 1
value: '2'	format:'2.00'	is_null: 0


Actual result:
--------------
value: 0	format:'0.00'	is_null: 0
value: ''	format:'0.00'	is_null: 0
value: null	format:'0.00'	is_null: 0
value: 'dummy'	format:'0.00'	is_null: 0
value: '2'	format:'2.00'	is_null: 0