Did some digging around and finally got it to work.
"@SQL=urn:schemas-microsoft-com:office:office#Keywords <> 'Yearly' AND urn:schemas:httpmail:subject LIKE '%Course Invoices%'"
This seems to be working for me.
From here:
https://forum.uipath.com/t/using-get-outlook-mail-messages-activity-filter-for-category-is-blank-or-unassigned/324872/7
and then to here
https://forum.uipath.com/t/using-get-outlook-mail-messages-activity-filter-for-category-is-blank-or-unassigned/324872/8?u=anup_rao