protocols: Handle the case when a weak protocol descriptor symbol is undefined

[hot3eed]

Background An externally-defined protocol that's imported as a weak symbol (e.g. for backward compatibility) might be undefined on some systems. Currently the bridge would crash in this case since it's trying to dereference a null pointer.

Changes

• Add an internal API for extracting a protocol name from its demangled conformance descriptor name.
• Handle a null protocol descriptor pointer.

How it was tested A hello-world SwiftUI app built using Xcode 15.0 (15A240d) on macOS 13.5.2 that was crashing before this change.

[hot3eed]

@oleavr Thanks for the review! Any other comments you'd like to be addressed?

[oleavr]

@hot3eed All good, just me being bad at context-switching — sorry for the delay! 😅