Thanks to everyone who replied.
I understand that the where clause is malformed but I don't know where it's coming from.
The GlobalID field is an Esri GlobalID so it is virtually impossible for it to be NULL.
I want to say that I would not formulate a query comparing a GlobalID to a string, but I guess this is not impossible. So I checked all the views in the database, and I checked all the layers in the map service for definition queries. I also checked the SQL Server Agent Jobs that work on the feature classes and tables contained in that feature service. Cannot find any where GlobalID="null" clauses.
This seems to be coming from the Esri innards. I have two attribute rules using the intersect operator in the web app that uses this feature service but they only refer to two text fields.
The feature classes and tables in the feature service are not versioned.
The malformed where clause is in the IIS log, so it is being sent from the web application (Web App Builder custom application) to the ArcGIS Server.
I really have no idea where that where clause is coming from.