Property can be created and drag in VFX editor even if the default VFX asset is Locked.

Steps to repro:
1. Create a new HDRP project.
2. GameObject > Visual Effects > Visual Effect.
3. Inspector > Assign Any default VFX Graph.
4. Toggle Blackboard button.
5. Create a property in Blackboard window.

Actual Result:
Locked assets are getting editable in default VFX graph.

Expected Result:
Locked assets should not get editable in default VFX graph.

Reproducible in:
2020.2.0a17, 2020.1.0b15, 2019.4.3f1, 2019.3.2f1 with Package Version 9.0.0-preview.33, 8.1.0, 8.0.1, 7.3.1.

Environment:
Occurring on Windows 10 & Mac 10.15.