fix: add an empty array fallback for fields with multiple documents type (#300)

Author: pdrvsky

packages/prime-field-document/src/PrimeFieldDocument.ts

@@ -76,7 +76,7 @@ export class PrimeFieldDocument extends PrimeField {
           locale: { type: GraphQLString },
         },
         resolve: async (root, args, ctx, info) => {
-          const values = root[this.schemaField.name];
+          const values = root[this.schemaField.name] || [];
           return Promise.all(
             values.map(value => {
               const [schemaId, documentId] = value.split(',');