[Bug Report]subscription returns null if query contains certain field

### What happened?

```graphql
type Mutation {
post(text: String!): Message!
}
type Subscription {
messageAdded: Message!
}

type Message {
id: ID!
text: String!
testMessage: String!
}
```

``` golang
type Message struct {
ID string `json:"id" gorm:"primary_key:true;type:char(36);column:id"`
Text string `json:"text"`

TestMessage *string `json:"test_message"`
}
```

As schema file shows, post messages will show up in "messageAdded" subscription.

But if subscription queried with `testMessage` field, the result will return `null` as below

And if subscription queried without `testMessage` field, the result seems to be normal


### Minimal graphql.schema and models to reproduce

I made an example project to debug
https://github.com/fredliang44/debug-gqlgen

### versions
- `gqlgen version`?
v0.11.3
- `go version`?
go1.14.1
- dep or go modules?
go mod

问题出在 gqlgen 库对 subscription 的处理上：当查询中包含 nullable 字段（testMessage）时会出现问题。首先检查 https://github.com/fredliang44/debug-gqlgen 中的示例项目，以重现该 bug。查看 subscription resolver 代码以及字段的解析方式，尤其是 nullable 指针的处理。检查为 Message 类型和 subscription 字段生成的代码。运行示例并在 resolver 周围添加 logging，有助于确定 null 是在哪里引入的。

由索引模型根据 Issue 内容生成。