Please use the <nullable> element to control this via XML. It's the same property, just with a different name from what the UI shows.

The XML schema tableInfo.xsd defines the properties. You can find it in the Subversion source code under:

schemas\tableInfo.xsd

or refer to HTML-based documentation for the <column> element (generated from the XSD itself) here:

https://www.labkey.org/download/schema-docs/xml-schemas/schemas/tableInfo_xsd/complexTypes/ColumnType.html

Thanks,
Josh