When attempting to filter for blank values using the syntax "" on either the pending vendor invoice grid view or the "Advanced filter or sort" page, we are seeing inconsistent and incorrect behavior.
The filter string "" returns records that have an empty string value. However, for foreign key fields (i.e. lookup fields and reference group fields), an empty value is often represented as NULL, which currently cannot be filtered for. Adding support for this may be considered for a future update.
String examples: purchase order and Product receipt in the pending vendor invoices filters.