Add check for "null" - string for productTypeReference key attribute

Author: salander85

src/main/java/com/commercetools/sync/producttypes/helpers/ProductTypeBatchValidator.java

@@ -167,7 +167,7 @@ private static String getProductTypeKey(@Nonnull final AttributeNestedType neste
       throws InvalidReferenceException {
 
     final String key = nestedAttributeType.getTypeReference().getId();
-    if (isBlank(key)) {
+    if (isBlank(key) || "null".equals(key)) {
       throw new InvalidReferenceException(BLANK_ID_VALUE_ON_REFERENCE);
     }
     return key;

src/test/java/com/commercetools/sync/producttypes/helpers/ProductTypeBatchValidatorTest.java

@@ -264,7 +264,7 @@ void validateAndCollectReferencedKeys_WithEmptyDraft_ShouldHaveEmptyResult() {
                     attributeTypeBuilder
                         .nestedBuilder()
                         .typeReference(
-                            productTypeReferenceBuilder -> productTypeReferenceBuilder.id("")))
+                            productTypeReferenceBuilder -> productTypeReferenceBuilder.id("null")))
             .name("invalidNested")
             .label(ofEnglish("koko"))
             .isRequired(true)
@@ -282,7 +282,7 @@ void validateAndCollectReferencedKeys_WithEmptyDraft_ShouldHaveEmptyResult() {
                                     .nestedBuilder()
                                     .typeReference(
                                         productTypeReferenceBuilder ->
-                                            productTypeReferenceBuilder.id(""))))
+                                            productTypeReferenceBuilder.id("null"))))
             .name("setOfInvalidNested")
             .label(ofEnglish("koko"))
             .isRequired(true)