Catch webmail-url empty but webmail configured and force to default

Quite some users managed to delete the contents of the webmail-url field in
setup, which forces front into a restart loop. Catch the case where a webmail
service is configured, but url is empty — and force to default /webmail.

closes #856

setup/flavors/compose/mailu.env

@@ -116,7 +116,11 @@ WEBROOT_REDIRECT=/webmail
 WEB_ADMIN={{ admin_path }}
 
 # Path to the webmail if enabled
+{% if webmail_type != 'none' and webmail_path == '' %}
+WEB_WEBMAIL=/webmail
+{% else %}
 WEB_WEBMAIL={{ webmail_path }}
+{% endif %}
 
 # Website name
 SITENAME={{ site_name }}