Fix YAML serialization of empty matrix_synapse_federation_domain_whitelist

We've previously changed a bunch of lists in `homeserver.yaml.j2`
to be serialized using `|to_nice_yaml`, as that generates a more
readable list in YAML.

`matrix_synapse_federation_domain_whitelist`, however, couldn't have
been changed to that, as it can potentially be an empty list.

We may be able to differentiate between empty and non-empty now
and serialize it accordingly (favoring `|to_nice_yaml` if non-empty),
but it's not important enough to be justified. Thus, always
serializing with `|to_json`.

Fixes #78 (Github issue)

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

@@ -120,8 +120,8 @@ use_presence: {{ matrix_synapse_use_presence|to_json }}
 #  - nyc.example.com
 #  - syd.example.com
 {% if matrix_synapse_federation_domain_whitelist is not none %}
-federation_domain_whitelist:
-{{ matrix_synapse_federation_domain_whitelist|to_nice_yaml }}
+{# Cannot use `|to_nice_yaml` here, as an empty list does not get serialized properly by it. #}
+federation_domain_whitelist: {{ matrix_synapse_federation_domain_whitelist|to_json }}
 {% endif %}
 
 # List of ports that Synapse should listen on, their purpose and their