AutoTimestampEventListener should only set dateCreated if it is null

Currently it overwrites whatever value is in the field. This complicates integration testing as it makes it hard to populate the database with known data using GORM.

Locate the AutoTimestampEventListener entry point in the GORM-related code and inspect how it handles dateCreated. Add coverage for an already populated dateCreated value, then verify that the existing value is preserved while a null value is still assigned a timestamp.

Written by the indexing model from the issue text.