Bug: incorrect Flow types for plural fragments

Hello! I think I found an error in generated Flow types. Fragments like this:

```graphql
fragment Ticket_passengers on Passenger @relay(plural: true) {
id
...MyFragment
}
```

currently generate this Flow type:

```js
export type Ticket_passengers = $ReadOnlyArray<{|
+id: string,
+$fragmentRefs: MyFragment$ref
+$refType: Ticket_passengers$ref,
|}>;
```

However, most of our types are nullable. This is not problem for most of the cases because we can just import the type and prefix it with `?`. But we cannot do it here because it should be:

```js
export type Ticket_passengers = $ReadOnlyArray;
```

I am afraid it is not possible to do it from outside when imported. It's unfortunate because parrent component defines this type:

```js
+passengers: ?$ReadOnlyArray
```

and as you can see these types are incompatible (inner object is correctly nullable here).