Make it possible to install maubot against Conduit

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3556

group_vars/matrix_servers

@@ -2577,7 +2577,7 @@ matrix_bot_maubot_homeserver_secret: |-
     {
       'synapse': matrix_synapse_registration_shared_secret | default(''),
       'dendrite': matrix_dendrite_client_api_registration_shared_secret | default(''),
-    }[matrix_homeserver_implementation]
+    }[matrix_homeserver_implementation] | default('')
   }}
 
 matrix_bot_maubot_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"