Add full function type when converting FnPtr::null

Author: lucic71

cpp2rust/converter/models/converter_refcount.cpp

@@ -1966,7 +1966,10 @@ std::string ConverterRefCount::GetDefaultAsString(clang::QualType qual_type) {
   if (qual_type->isPointerType()) {
     auto pointee_type = qual_type->getPointeeType();
     if (pointee_type->isFunctionType()) {
-      ret = "FnPtr::null()";
+      auto *proto = pointee_type->getAs<clang::FunctionProtoType>();
+      assert(proto && "Function pointer default without a prototype");
+      ret =
+          std::format("FnPtr::<{}>::null()", ConvertFunctionPointerType(proto));
     } else {
       if (pointee_type->isVoidType()) {
         ret = "AnyPtr::default()";