fix(api): incorrect selection set for optional associated field

[lawmicha]

Issue #, if available:
#483

Description of changes:
This PR fixes a problem with the selection set that is generated for a Model containing a connection to another Model. Take this for example

type Post @model {
   user: User @connection
}

translates to "A Post has an optional association to the User"
.belongsTo(post.user, is: .optional, ofType: User.self, targetName: "postUserId"),

The original code checked for required associations

let isRequiredAssociation = field.isRequired && field.isAssociationOwner
if isRequiredAssociation, let associatedModel = field.associatedModel {

Consider a case where we retrieve the Post and the User does not exist on the Post. The selection set's intent is to retrieve that data in the response, and no User data is returned, thus nothing gets deserialized into the Post's User field.

This PR adds the following schema to test building a GraphQLRequest

type User @model {
    id: ID!
    name: String!
    following: [UserFollowing] @connection (name: "following")
    followers: [UserFollowers] @connection (name: "followers")
}

type UserFollowing @model {
  id: ID!
  user: User @connection(name: "following")
  followingUser: User @connection
}

type UserFollowers @model {
  id: ID!
  user: User @connection(name: "followers")
  followersUser: User @connection
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.