Do not add federation_sender_instances/pusher_instances configuration if not necessary

This keeps the configuration cleaner and hopefully prevents odd issues
if Synapse (by any chance) happens to interpret `pusher_instances: []`
as "no pushers", instead of as "push from the master process".

I haven't confirmed what an empty pushers/federation-senders list means,
so it's safer to just avoid it.

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

@@ -2851,9 +2851,13 @@ send_federation: {{ matrix_synapse_send_federation | to_json }}
 #
 #federation_sender_instances:
 #  - federation_sender1
+{% if matrix_synapse_federation_sender_instances | length > 0 %}
 federation_sender_instances: {{ matrix_synapse_federation_sender_instances | to_json }}
+{% endif %}
 
+{% if matrix_synapse_federation_pusher_instances | length > 0 %}
 pusher_instances: {{ matrix_synapse_federation_pusher_instances | to_json }}
+{% endif %}
 start_pushers: {{ matrix_synapse_start_pushers | to_json }}
 
 # When using workers this should be a map from `worker_name` to the