See the following fields (each of which are foreign keys) that define themselves as nullable:

1. customfield_category.contextid;
2. customfield_field.categoryid - this can be ignored, handlers may not support categories;
3. customfield_data.contextid

Investigate whether they should be nullable or not. In the first case (the customfield_category table) - ensure we cleanup any currently null data that may exist

Found while looking at MDL-83094