dorm.adapter.Sqlite3 ignores nullable columns

[skamradt]

The nullable attribute is ignored on columns when performing an update or insert.

I believe the simplest path to get this working is to modify SetSqlite3ParameterValue to also pass the current field mapping and a similar substitution made for null as what is done for an empty blob.

In the test case that I came up with, I have an optional column which is a string and indexed unique. Not allowing empty string to be nulls would mean that the field is no longer optional and would generate a key violation.