Do not define `url_preview_ip_range_blacklist` in Synapse's `homeserver.yaml` if URL preview is disabled

It's useless and just makes the config longer.

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

@@ -1131,7 +1131,9 @@ url_preview_enabled: {{ matrix_synapse_url_preview_enabled|to_json }}
 #
 # Note: The value is ignored when an HTTP proxy is in use
 #
+{% if matrix_synapse_url_preview_enabled %}
 url_preview_ip_range_blacklist: {{ matrix_synapse_url_preview_ip_range_blacklist | to_json }}
+{% endif %}
 
 # List of IP address CIDR ranges that the URL preview spider is allowed
 # to access even if they are specified in url_preview_ip_range_blacklist.