Make item encryption key optional for collections/items

Collections still have a unique encryption key (their collection key), and items
just have a unique key per item in a collection that's derived from the main key
and if we ever want to share items across collections or do something fancy like
that we can just add an encrypted key in there.

django_etebase/serializers.py

@@ -116,7 +116,7 @@ class CollectionItemRevisionSerializer(serializers.ModelSerializer):
 
 
 class CollectionItemSerializer(serializers.ModelSerializer):
-    encryptionKey = BinaryBase64Field()
+    encryptionKey = BinaryBase64Field(required=False, default=None)
     etag = serializers.CharField(allow_null=True, write_only=True)
     content = CollectionItemRevisionSerializer(many=False)
 
@@ -186,7 +186,7 @@ class CollectionSerializer(serializers.ModelSerializer):
     stoken = serializers.CharField(read_only=True)
 
     uid = serializers.CharField(source='main_item.uid')
-    encryptionKey = BinaryBase64Field(source='main_item.encryptionKey')
+    encryptionKey = BinaryBase64Field(source='main_item.encryptionKey', required=False, default=None)
     etag = serializers.CharField(allow_null=True, write_only=True)
     version = serializers.IntegerField(min_value=0, source='main_item.version')
     content = CollectionItemRevisionSerializer(many=False, source='main_item.content')