message: [Swift] Fix codegen for optional list of non-optional elements
  Fixes
diff: @@ -245,27 +245,28 @@ export class Helpers {
  }
  if (type instanceof GraphQLList) {
  + const elementType = type.ofType;
  if (isOptional) {
  return `${expression}.flatMap { ${makeClosureSignature(
  this.typeNameFromGraphQLType(type, inputTypeName, false),
  this.typeNameFromGraphQLType(type, outputTypeName, false)
  - )} value.map { ${this.mapExpressionForType(
  - type.ofType,
  + )} value.map { ${makeClosureSignature(
  + this.typeNameFromGraphQLType(elementType, inputTypeName),
  + this.typeNameFromGraphQLType(elementType, outputTypeName)
  + )} ${this.mapExpressionForType(
  + elementType,
  undefined,
  makeExpression,
  - `${makeClosureSignature(
  - this.typeNameFromGraphQLType(type.ofType, inputTypeName),
  - this.typeNameFromGraphQLType(type.ofType, outputTypeName)
  - )} value`,
  + 'value',
  inputTypeName,
  outputTypeName
  )} } }`;
  } else {
  return `${expression}.map { ${makeClosureSignature(
  - this.typeNameFromGraphQLType(type.ofType, inputTypeName),
  - this.typeNameFromGraphQLType(type.ofType, outputTypeName)
  + this.typeNameFromGraphQLType(elementType, inputTypeName),
  + this.typeNameFromGraphQLType(elementType, outputTypeName)
  )} ${this.mapExpressionForType(
  - type.ofType,
  + elementType,
  undefined,
  makeExpression,
  'value',
  