update lib to return none (#390)

## Issue


## Solution

---------

Co-authored-by: Mehdi Bendriss <bendrissmehdi@gmail.com>

lib/charms/mongodb/v0/config_server_interface.py

@@ -42,7 +42,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 10
+LIBPATCH = 11
 
 
 class ClusterProvider(Object):
@@ -378,7 +378,7 @@ def get_tls_statuses(self) -> Optional[StatusBase]:
             )
             return BlockedStatus("mongos CA and Config-Server CA don't match.")
 
-        return
+        return None
 
     def get_config_server_name(self) -> Optional[str]:
         """Returns the name of the Juju Application that mongos is using as a config server."""