When an empty array is given to a Zend_Filter_Input for validation, it's considered as not empty and isValid() method returns true.

Here is a simple snippet that reproduce the bug :

<?php
require_once 'Zend/Filter/Input.php';
require_once 'Zend/Validate/NotEmpty.php';
 
$input = new Zend_Filter_Input(null, array('test' => array()));
$input->setData(array('test' => array()));

if ($input->isValid()) {
    echo "Array is not empty\n";
} else {
    echo "Array is empty\n";
}

$input->setData(array('test' => ''));

if ($input->isValid()) {
    echo "String is not empty\n";
} else {
    echo "String is empty\n";
}

It will display : Array is not empty String is empty

As Zend_Validate_NotEmpty, Zend_Filter_Input should considers that an empty array is an empty value, and is not valid.