If you pass an invalid borrowernumber (ie a borrowernumber that doesn't exist in the borrowers table) to GetMemberDetails, you get the following information in the hashref it returns: 'authflags' => undef, 'flags' => {}, 'amountoutstanding' => '0', 'showname' => undef It should just return 'undef' so you know that the borrower you're looking for doesn't exist in the database.
GetMemberDetails no longer exists in Koha.