replace null with empty enum set

in addition to the fix in pr #144

CHANGELOG.md

@@ -8,7 +8,7 @@ Changelog
 * fix some invalid filters in the default swagger examples ([#111])
 * fix returning invalid GeoJSON using empty coordinates for deletion contributions ([#129], [#131])
 * fix using a proper boolean data type instead of a string for contributionType in response ([#135])
-* fix NPE with createOSMFeature ([#141])
+* fix a null pointer exception in data extraction requests ([#141], [#145])
 * make sure geometry filters are applied to all returned features of elementsFullHistory requests ([#109])
 
 ### Performance and Code Quality
@@ -37,6 +37,7 @@ Changelog
 [#135]: https://github.com/GIScience/ohsome-api/pull/135
 [#138]: https://github.com/GIScience/ohsome-api/issues/138
 [#141]: https://github.com/GIScience/ohsome-api/issues/141
+[#145]: https://github.com/GIScience/ohsome-api/pull/145
 
 
 ## 1.3.2

src/main/lombok/org/heigit/ohsome/ohsomeapi/executor/DataExtractionTransformer.java

@@ -2,6 +2,7 @@
 
 import java.io.Serializable;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -173,9 +174,9 @@ public List<Feature> buildUnchangedFeatures(OSMEntitySnapshot snapshot) {
     properties.put("@validTo", endTimestamp);
     boolean addToOutput = addEntityToOutput(entity, geom);
     if (addToOutput) {
-      return Collections.singletonList(
-          exeUtils.createOSMFeature(entity, geom, properties, keysInt, includeTags,
-              includeOSMMetadata, isContributionsEndpoint, elementsGeometry, null));
+      return Collections.singletonList(exeUtils.createOSMFeature(entity, geom, properties,
+          keysInt, includeTags, includeOSMMetadata, isContributionsEndpoint, elementsGeometry,
+          EnumSet.noneOf(ContributionType.class)));
     } else {
       return Collections.emptyList();
     }