client:codegen uses Flow "maybe" type for nullable fields, which allows undefined

Generated types for nullable (or non-required) fields use the Flow "maybe" type. This forces the client to do unnecessary type refinements.

For example, the following GraphQL type:

```
type Foo {
bar: String
baz: String!
}
```

...generates the following Flow type:

```
{
__typename: "Foo",
bar: ?string,
baz: string,
};
```

This is wrong because "maybe" types are allowed to be `undefined` in addition to `null` and the indicated type:

https://flow.org/en/docs/types/maybe/

I think the correct Flow type should be this:

```
{
__typename: "Foo",
bar: string | null,
baz: string,
};
```

**Versions**

apollo@^2.18.3
apollo-codegen-flow@^0.33.28

Start in the apollo-codegen-flow generator entry point that emits nullable field types and locate its existing Flow code-generation tests. Reproduce the Foo example, then verify that nullable or non-required fields produce a type allowing null but not undefined, while required fields remain unchanged.

Written by the indexing model from the issue text.