php-mysql多字段搜索空字段

<?php
if(isset($_POST['submit'])) {
    $fields = array('field1', 'field2', 'field3');
    $conditions = array();
    foreach($fields as $field){
        if(isset($_POST[$field]) && $_POST[$field] != '') {
            $conditions[] = "`".$field."` like '%" . mysql_real_escape_string($_POST[$field]) . "%'";
        }
    }
    $query = "SELECT * FROM customer ";
    if(count($conditions) > 0) {
        $query .= "WHERE " . implode (' AND ', $conditions); 
    }
    $result = mysql_query($query); 
    $say = mysql_num_rows($result);
    if ($say == 0) {
        echo "<tr>no result.</tr>";
        } else {
echo '...';     
    while($row = mysql_fetch_array($result))
  {
  ...
  }}
} ?>

为什么这个代码不检查空字段？即使表单提交为空，它也会返回字段为空的结果。