Fix "object of type 'NoneType' has no len()" error

Fixup for 5eff67371d - https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1884

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1049,10 +1049,10 @@ media_store_path: "/matrix-media-store-parent/{{ matrix_synapse_media_store_dire
 max_upload_size: "{{ matrix_synapse_max_upload_size_mb }}M"
 
 media_retention:
-{% if matrix_synapse_media_retention_local_media_lifetime|length > 0 %}
+{% if matrix_synapse_media_retention_local_media_lifetime %}
   local_media_lifetime: {{ matrix_synapse_media_retention_local_media_lifetime|to_json }}
 {% endif %}
-{% if matrix_synapse_media_retention_remote_media_lifetime|length > 0 %}
+{% if matrix_synapse_media_retention_remote_media_lifetime %}
   remote_media_lifetime: {{ matrix_synapse_media_retention_remote_media_lifetime|to_json }}
 {% endif %}