Specify Skip Rule Syntax in BPM to Skip Approval When Requester is null value

Summary:

Is there a way to use the "Specify Skip Rule" to skip an invoice approval block of rules when the requester field at the invoice header area is left blank?

I've tried numerous variations using this section but not sure what syntax it should be and I can't find a list of acceptable functions:

requester/task:task/task:payload/ns2:getInvoiceHeader1Response/ns2:result/ns1:RequesterName is null

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