if语句- PHP获取空参数

我从html调用PHP函数，我想确保一件事发生时，相应的输入框是空的，其他的事情时不是。由于某些原因，PHP认为它总是不为空。我不知道为什么。

<input type="button" 
value="checkout"
onClick="self.location='testphp.php?checkout=true&folderName=''' +  document.folderForm.folderLocation.value + ''''">
</button> 
<form enctype="multipart/form-data" method="post" name="folderForm">
<input type=file name="myfile">
<input type=text name="folderLocation" id="folderLocation">
<input type=hidden name="folderName">
<input type=button value="Get Folder" onclick="javascript:GetFolder();">
<input type=hidden name=submitted value=1> 
<input type="submit" value="Create Repository" />
</form>

if($_GET['checkout']){
    if( isset( $_GET['folderName'] ) && !empty( $_GET['folderName'] ) ) {
        echo $_GET["folderName"];
        echo exec("/var/www/html/checkout.sh ".$_GET["folderName"]);
    } else {
        echo "<script type='text/javascript'>alert('Please choose repository from the list first');</script>";  
    }
}

任何想法?