[#333] fix drep type detection

[jankun4]

List of changes

Fix

• fixed problems with drep type detection when changing metadata from not null to null

Checklist

• 333
• My changes generate no new warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the changelog
• I have added tests that prove my fix is effective or that my feature works