Add missing import

Author: abarto

mongoengine/context_managers.py

@@ -1,6 +1,7 @@
 from contextlib import contextmanager
 
 from pymongo.write_concern import WriteConcern
+from pymongo.read_concern import ReadConcern
 from six import iteritems
 
 from mongoengine.common import _import_class
@@ -263,10 +264,14 @@ def set_write_concern(collection, write_concerns):
 @contextmanager
 def set_read_write_concern(collection, write_concerns, read_concerns):
     combined_write_concerns = dict(collection.write_concern.document.items())
-    combined_write_concerns.update(write_concerns)
+
+    if write_concerns is not None:
+        combined_write_concerns.update(write_concerns)
 
     combined_read_concerns = dict(collection.read_concern.document.items())
-    combined_read_concerns.update(read_concerns)
+    
+    if read_concerns is not None:
+        combined_read_concerns.update(read_concerns)
 
     yield collection.with_options(
         write_concern=WriteConcern(**combined_write_concerns),