fix: Prevent default values for offer time fields when unnecessary

[lotharking]

Summary

Changes

• Updated the @Transform decorator to ensure offerExpirationTime and offerStartTime are not assigned default values. The transformation now checks the value's existence before parsing it with DateParser, avoiding unintended field inclusion.

Related Issues

Testing

Checklist

• I have commented on my code, especially in areas that are hard to understand.
• I have added only changes relevant to the issue in question.
• I have added tests to confirm that my fix is effective or that my feature works as intended.
• I have modified existing tests as needed to accommodate my changes.
• I have flagged specific areas for further review, if necessary.
• I have updated the documentation where necessary.