fix: createdAt should be optional field

Description

Make createdAt optional field, as it is not aways populated.

Related issues

Testing

Checklist

• I have updated the documentation (if necessary)
• I have added unit tests for new functionality (if applicable)
• My code follows the style guidelines of this project
• I have tested this code on my local machine before submitting this MR

Reviewers

@boyan.tsolov @alexey.lunin

Additional Notes

Closes #58 (closed)