Error when synchronizing from QField when an "int" field has been cleared in QField

[gioman]

Qfield Sync:
4.0.0-BETA8
v3.4.4

QField:
2.0.2 Beta3

QGIS:
master
3.20

Take a project with a PostGIS layer with an "int" field (filled), make it offline and port it to QField.

In QField edit an existing feature and clear the "int" field.

Try synchronize basck to PostGIS with QFieldSync, it will fail as it will try to update the feature with something like

UPDATE "schemaname"."tablename" SET "columnname"='' WHERE "i...

instead of something like

UPDATE "schemaname"."tablename" SET "columnname"=NULL WHERE "i...

[paschendale]

The same thing happens on numeric and timestamp fields as well.