Remove KeyPair deserialization (#6395)
Remove KeyPair deserialization (#6395)

Fixes #6389

Context
Strong Name Key Pairs aren't available on .NET 6+, and although it's fine to serialize null and remember that's what it is, deserializing involves assigning null to something that shouldn't exist, which is throwing an error, invalidating all RAR caches.

This fixes that problem.

Changes Made
No longer serialize or deserialize a null value for KeyPair.

Testing
Allowed precomputed cache to function properly. (Local test only)
AndyGerlicher/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,rainersigwald/msbuild