How to reproduce:
1. Create a new Unity project
2. Open the Package Manager (Window -> Package Management -> Package Manager)
3. Click the "+" button -> Create package 
4. Go to Manage -> Edit Manifest
5. In the Inspector, go to the "Minimum Unity version" section -> "Release" field
6. Delete everything and click Apply

Expected results: The applied changes leave that field empty and remove it from the manifest as well
Actual results: Release can be left empty and Package Manager defaults to the "0a1" version

Reproducible with: 6000.3.0a4
Can't test with: 6000.0.55f1, 6000.1.15f1, 6000.2.0f1 (no such functionality)

Reproducible on: Windows 11 Pro
Not reproducible on: No other environment tested