Our customers / product requires the ability to filter by null/blank guid fields.

For example, the Invoices endpoint has a RepeatingInvoiceID field. We have customers that need to find all invoices that DID or DID NOT originate from a repeating invoice.

The logical approach would be via the where clause using one of the following syntax.

?where=RepeatingInvoiceID != null
?where=RepeatingInvoiceID == null
?where=RepeatingInvoiceID != ''
?where=RepeatingInvoiceID == ''

This not only needs to apply to invoices but all endpoints (e.g. the invoices BrandingThemeID, the bank transactions OverpaymentID or PrepaymentID, etc.)