Handle empty labels correctly.

roles/custom/matrix-element-call/templates/element-call-labels.j2

@@ -39,6 +39,8 @@ traefik.http.routers.element-call-router.middlewares={{ middlewares | join(',')
 {% endif %}
 
 # Additional labels (if any) specified by the user
+{% if matrix_element_call_container_extra_arguments.keys() | length > 0 %}
 {% for key, value in matrix_element_call_container_extra_arguments.items() %}
 {{ key }}={{ value }}
 {% endfor %}
+{% endif %}