Not existing neighbor return None, not empty dict

taiga/projects/issues/serializers.py

@@ -81,7 +81,9 @@ class IssueListSerializer(IssueSerializer):
 
 class IssueNeighborsSerializer(NeighborsSerializerMixin, IssueSerializer):
     def serialize_neighbor(self, neighbor):
-        return NeighborIssueSerializer(neighbor).data
+        if neighbor:
+            return NeighborIssueSerializer(neighbor).data
+        return None
 
 
 class NeighborIssueSerializer(serializers.ModelSerializer):

taiga/projects/tasks/serializers.py

@@ -79,7 +79,9 @@ class TaskListSerializer(TaskSerializer):
 
 class TaskNeighborsSerializer(NeighborsSerializerMixin, TaskSerializer):
     def serialize_neighbor(self, neighbor):
-        return NeighborTaskSerializer(neighbor).data
+        if neighbor:
+            return NeighborTaskSerializer(neighbor).data
+        return None
 
 
 class NeighborTaskSerializer(serializers.ModelSerializer):

taiga/projects/userstories/serializers.py

@@ -113,7 +113,9 @@ class UserStoryListSerializer(UserStorySerializer):
 
 class UserStoryNeighborsSerializer(NeighborsSerializerMixin, UserStorySerializer):
     def serialize_neighbor(self, neighbor):
-        return NeighborUserStorySerializer(neighbor).data
+        if neighbor:
+            return NeighborUserStorySerializer(neighbor).data
+        return None
 
 
 class NeighborUserStorySerializer(serializers.ModelSerializer):