Issue with nullable reference when using federation

[mwoelk]

Describe the bug
When using a nullable @Reference, there are two small issues.

• The 4th argument of the @Reference decorator is used to set the relation to nullable. But only passing {nullable: true} leads to a type error because it also expects a key property which should have also been omitted here:
  

  nestjs-query/packages/query-graphql/src/decorators/reference.decorator.ts

  Line 9 in 70fd48e

  export type ReferenceDecoratorOpts<DTO, Relation> = Omit<ResolverRelationReference<DTO, Relation>, 'DTO'>;

• Even if the referencedId is null, an object is returned with a typename and a null reference.
  

  nestjs-query/packages/query-graphql/src/resolvers/relations/references-relation.resolver.ts

  Lines 36 to 39 in 70fd48e

  	[`${baseNameLower}Reference`](@Parent() dto: DTO): RepresentationType {
  	// eslint-disable-next-line @typescript-eslint/naming-convention
  	return { __typename: baseName, ...pluckFields<DTO, Relation>(dto, reference.keys) };
  	}

  
  It should return null otherwise we get the error that the id field on the referenced type is not nullable!

Have you read the Contributing Guidelines?

yes

To Reproduce
Steps to reproduce the behavior:

1. Use federation
2. Have one DTO define a reference of another external DTO that can be nullable

Expected behavior
In case of a nullable reference, the resolver should return null instead of a typename with an id of null.

Desktop (please complete the following information):

• Node Verson [14.13.1]
• Nestjs-query Version [v0.25.1]