ERP5: set the default backend-path parameter value to empty string

The previous value, '/', caused problems: some generated urls had two
consecutive slashes, which can be misinterpreted by the waitress server.

stack/erp5/buildout.hash.cfg

@@ -78,7 +78,7 @@ md5sum = d41d8cd98f00b204e9800998ecf8427e
 
 [template-erp5]
 filename = instance-erp5.cfg.in
-md5sum = ff2f4d9f806fa783421d34204f2a74c8
+md5sum = d2221cab154dd0cc6c21527c435f7126
 
 [template-zeo]
 filename = instance-zeo.cfg.in

stack/erp5/instance-erp5.cfg.in

@@ -203,7 +203,7 @@ software-type = zope
 {%   set section_name = 'request-' ~ partition_name -%}
 {%   set zope_family = zope_parameter_dict.get('family', 'default') -%}
 {%   do zope_family_name_list.append(zope_family) %}
-{%   set backend_path = zope_parameter_dict.get('backend-path', '/') % {'site-id': site_id} %}
+{%   set backend_path = zope_parameter_dict.get('backend-path', '') % {'site-id': site_id} %}
 {#   # default jupyter zope family is first zope family. -#}
 {#   # use list.append() to update it, because in jinja2 set changes only local scope. -#}
 {%   if not jupyter_zope_family_default -%}