Fix relation snippet in why-nullable

[benjie]

https://www.graphile.org/postgraphile/why-nullable/

Currently the snippet (incorrectly) marks all relations as non-nullable. Should only apply if the columns themselves are non-nullable.

Something like this: (needs testing)

module.exports = function NonNullRelationsPlugin(builder) {
  builder.hook('GraphQLObjectType:fields:field', (field, build, context) => {
    const { isPgForwardRelationField, pgFieldIntrospection } = context.scope;
    if (isPgForwardRelationField) {
      const linkedAttributeNums = pgFieldIntrospection.keyAttributeNums;
      const relationIsNotNull = pgFieldIntrospection.class.attributes.filter(
        attr => linkedAttributeNums.indexOf(attr.num) >= 0
      ).every(attr => attr.isNotNull || attr.type.domainIsNotNull);

      if (relationIsNotNull) {
        return {
          ...field,
          type: new build.graphql.GraphQLNonNull(field.type),
        };
      }
    }

    return field;
  });
};

HT @enisdenjo