Currently it's not possible to patch/update nullable fields in a collection altough this is supported on a datasource.
You can collect data from a datasource table with nullable columns into a collection but you will not be able to update those fields. (also no error is thrown)
This makes an offline scenario with tables that use nullable columns impossible.
The only solution, if you are allowed, is to modify those fields with empty values "" on the DB side when these are string or char fields but when you have columns with a data type like interger this is not a solution/workaround since the fields need to have a value set when Nulls are not allowed.