Assert compiling dotnet/runtime in NullableWalker

[jaredpar]

When compiling dotnet/runtime with the latest compiler in Debug we hit the following assert in the nullable walker (around line 7391).

            Debug.Assert(false); // If this assert fails, add an appropriate test.

The line of code which is causing the assert is the following:

https://github.com/dotnet/runtime/blob/f9ebd42d05ca88f086d472832487fbccfc7fe4ac/src/tools/illink/src/ILLink.Shared/TrimAnalysis/HandleCallAction.cs#L1166-L1168

Found while digging into dotnet/runtime#82467