Bad `func` slot type + SBCL 2.1.2 dropped support sb-sprof:map-trace-samples

The `func` slot of the `node` class has type `(or string sb-di::compiled-debug-fun)` but its initform is `nil`, which does not match.

This is an issue on modern SBCLs.

Either the initform needs to be set to `""` or `null` must be added to the types.

Look at the node class definition in the source code, likely in a file like src/ or flamegraph.lisp. The func slot's type declaration and initform need to be reconciled. Check SBCL 2.1.2 changes regarding sb-sprof:map-trace-samples. After modifying the type or initform, test by building and running the profiler on a simple Common Lisp program to ensure no type errors occur.

索引モデルが issue の本文から書いたものです。