Update join_token condition

Change-Id: I9b6c3e0739ef26741d00c2d6cbb30847fa708414
diff --git a/docker/swarm.sls b/docker/swarm.sls
index cd42f01..a6322f3 100644
--- a/docker/swarm.sls
+++ b/docker/swarm.sls
@@ -61,9 +61,7 @@
 {%- endif %}
 {%- endfor %}
 
-{%- if join_token %}
-
-{%- set join_token = swarm.get('join_token', {}).get(swarm.role, join_token[-1]) %}
+{%- set join_token = swarm.get('join_token', {}).get(swarm.role, join_token[-1] if join_token else 'unknown') %}
 
 docker_swarm_join:
   cmd.run:
@@ -80,5 +78,3 @@
 {%- endif %}
 
 {%- endif %}
-
-{%- endif %}