Allow renaming/editing a notification config. By replying with
a successful GetBucketNotification response, without checking
for any missing config ARN in targetList.
Fixes#7650
fixes a regression introduced in 0e4431725cd85236d3d18643f8fc2506c7d88c66
when removing a previously applied notification configuration.
event.ParseConfig() was stricter in terms of handling notification
configuration, we need to allow when notification configuration is
sent empty, this is the way to remove notification configuration.
