Dave, I created a similar odata service, and I added the settings: { support null; } I queried my service and I am able to query null values as:
url/entity?$filter=COLUMN eq null&$format=json
$format=json is to get a json format but not required.
my service shows a red underline on settings but it activated fine.. make sure you do not have a colon between support null (again, it should not have a colon)
the issue may be your HANA version - I believe the nulls are supported starting on version SP09