All Shadow override removes on resetting in Sky and Fog Volume.

Steps to repro:
1. Create a new HDRP template project
2. Hierarchy > Sky & Fog Volume > Inspector.
3. Add Override > Shadowing > Contact Shadows.
4. Enable & edit the properties > Reset.

Actual Result:
Contact Shadow override removes on resetting in Sky and Fog Volume.

Expected Result:
Contact Shadow override Should not remove on resetting in Sky and Fog Volume.

Reproducible in:
2018.2.0a18, 2020.1.0b16 with 8.2.0.

Working Fine in:
2020.2.0a18 With Package Version 9.0.0, 8.1.0.

Environment:
Occurring on Windows 10 & Mac 10.15.