fix: category reference resolver

Author: marcelogpinheiro

src/integration-test/java/com/commercetools/sync/integration/ctpprojectsource/categories/CategorySyncIT.java

@@ -459,15 +459,15 @@ void syncDrafts_fromCategoriesWithoutKeys_ShouldNotUpdateCategories() {
 
     final CategoryDraft oldCategoryDraft1 =
         CategoryDraftBuilder.of()
-            .name(LocalizedString.of(Locale.ENGLISH, "cat1"))
+            .name(LocalizedString.of(Locale.ENGLISH, key1))
             .slug(LocalizedString.of(Locale.ENGLISH, slug1))
             .key(key1)
             .custom(CategoryITUtils.getCustomFieldsDraft())
             .build();
 
     final CategoryDraft oldCategoryDraft2 =
         CategoryDraftBuilder.of()
-            .name(LocalizedString.of(Locale.ENGLISH, "cat2"))
+            .name(LocalizedString.of(Locale.ENGLISH, key2))
             .slug(LocalizedString.of(Locale.ENGLISH, slug2))
             .key(key2)
             .custom(CategoryITUtils.getCustomFieldsDraft())
@@ -483,6 +483,11 @@ void syncDrafts_fromCategoriesWithoutKeys_ShouldNotUpdateCategories() {
     final CategoryDraft newCategoryDraft2 =
         CategoryDraftBuilder.of(oldCategoryDraft2).key(null).build();
 
+    assertThat(oldCategoryDraft1.getKey()).isNotEqualTo(newCategoryDraft2.getKey());
+    assertThat(oldCategoryDraft2.getKey()).isNotEqualTo(newCategoryDraft2.getKey());
+    assertThat(oldCategoryDraft1.getSlug().get(Locale.ENGLISH)).isEqualTo(newCategoryDraft1.getSlug().get(Locale.ENGLISH));
+    assertThat(oldCategoryDraft2.getSlug().get(Locale.ENGLISH)).isEqualTo(newCategoryDraft2.getSlug().get(Locale.ENGLISH));
+
     final Category targetCat1 =
         TestClientUtils.CTP_TARGET_CLIENT
             .categories()

src/main/java/com/commercetools/sync/categories/helpers/CategoryReferenceResolver.java

@@ -189,6 +189,6 @@ public CompletableFuture<Map<String, String>> populateKeyToIdCachesForReferenced
     }
 
     return collectionOfFuturesToFutureOfCollection(futures, toList())
-        .thenApply(maps -> maps.get(0));
+        .thenApply(maps -> maps.isEmpty() ? java.util.Collections.emptyMap() : maps.get(0));
   }
 }