callback: guard against None values (#441)

See Azure/azure-sdk-for-python#11419 (comment) on how to get
progress information.
Also related: #275 (comment) and iterative/dvc#7704.

Invalid values should not be passed to callbacks.

adlfs/spec.py

@@ -82,7 +82,9 @@ def wrapper(response):
 
         current = response.context.get(key)
         total = response.context["data_stream_total"]
-        if not sent_total:
+        if current is None:
+            return
+        if not sent_total and total is not None:
             callback.set_size(total)
         callback.absolute_update(current)