Xpath function to find out string length of an attribute

I have xml message body as below. I need to find out if string length of the attribute 'SenderId' is greater than 0. I am unable to find this.

XML Message

<?xml version="1.0"?>
<Audit xmlns="http://schemas.Auditlog.xsd">
<MessageId>1234</MessageId>
<SenderDetails CreatedDate="2017-Jan-12" SenderId="App1"/>
</Audit>

I am using below xpath function to get the string length of SenderId

when(xpath("string-length(string(//node()[local-name()='SenderDetails']/@[local-name()='SenderId']/text()))>0))

But it is not working.