software/caddy-frontend: review caddy-log-access to not overwrite installed paths

We were using caddy-log-access-header to make sure we have at least one
file to include, but this was implemented in a way that the config file
was overwritten.
Reimplement this by using caddy-log-access-empty to create an empty file
when there are no slaves, caddy-log-access otherwise.

software/caddy-frontend/buildout.hash.cfg

@@ -30,7 +30,7 @@ md5sum = 087bd9404cd120bd7602a9fbfcddc064
 
 [template-slave-list]
 filename = templates/apache-custom-slave-list.cfg.in
-md5sum = cbe0ee673bf143cd4b874167b14ad22d
+md5sum = 29a61267959cc9ba7cdcd96fef41641a
 
 [template-slave-configuration]
 filename = templates/custom-virtualhost.conf.in

software/caddy-frontend/templates/apache-custom-slave-list.cfg.in

@@ -465,11 +465,11 @@ template = inline:
 
 rendered = ${:file}
 
-[caddy-log-access-header]
+[caddy-log-access-empty]
 # Caddy refuse to start if an `import`ed file is empty, so we prepend a header
 # so that the file is never empty.
 < = jinja2-template-base
-template = inline: # This file contain directives to serve directories with log files
+template = inline: # This file contain directives to serve directories with log files for shared instances, but no shared instances are defined yet.
 rendered = {{frontend_configuration.get('log-access-configuration')}}
 
 [buildout]
@@ -481,10 +481,12 @@ extends =
 parts +=
     kedifa-updater
     kedifa-updater-run
-    caddy-log-access-header
 {% for part in part_list %}
 {{ '    %s' % part }}
 {% endfor %}
+{% if 'caddy-log-access' not in part_list %}
+    caddy-log-access-empty
+{% endif %}
     publish-caddy-information
     tunnel-6to4-base-http_port
     tunnel-6to4-base-https_port