Search for documents with blank/null attribute value

I know that I can use this syntax to search for documents with a specific attribute value:

$PWDocuments = Get-PWDocumentsBySearch -FolderPath $MyFolderPath -Attributes @{ "My_Attribute"="Desired Value" }

My question is, how can I search for documents having a blank or null value for a particular attribute?  This does not return any documents, even though I know they exist:

$PWDocuments = Get-PWDocumentsBySearch -FolderPath $MyFolderPath -Attributes @{ "My_Attribute"="" }

Any suggestions would be appreciated.  Thanks!