Handle missing Network.framework peer certificate chain

[liveans]

Fixes crash in #123395

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Fixes a crash in Apple Network Framework TLS/WebSockets scenarios by safely handling cases where the peer certificate chain handle is missing/invalid, avoiding a NullReferenceException during remote certificate extraction.

Changes:

• Make the Network.framework peer chain handle nullable when retrieving the remote certificate.
• Early-return null when the chain handle is missing or invalid, preventing calls into AppleCrypto with a null handle.
• Ensure the copied certificate chain handle is disposed when it was created via SslCopyCertChain (SafeDeleteSslContext path), even on the early-return path.