FIELD_DEFINITION directive return nil obj

### What happened?

- FIELD_DEFINITION directive return nil obj

### What did you expect?

- return arguments

```graphql
directive @hello(reason: String = "No longer supported") on FIELD_DEFINITION

extend type Query {
test(hello: String!, world: String!): String! @hello
}
```

```go
config := gqlgen.Config{
Resolvers: &Resolver{service},
}
config.Directives.Hello = func(ctx context.Context, obj interface{}, next graphql.Resolver, reason *string) (res interface{}, err error) {
fmt.Printf("Obj %+v\n", obj)
return next(ctx)
}
```

output: nil

### versions
- `go run github.com/99designs/gqlgen version`: v0.17.36
- `go version`: v1.20.6

该 issue 涉及接收 nil obj 的 FIELD_DEFINITION 指令。查看 gqlgen 代码库中的指令实现，可能位于 internal/exec 或 internal/directives。检查 obj 是如何传递给 FIELD_DEFINITION 指令的。运行提供的示例以复现该问题。确保 obj 按预期包含父对象或参数，从而验证修复。

由索引模型根据 Issue 内容生成。