fix(conf) fallback for lack of value for var placeholders

`anonymous_reports=on` + `socket.toip` not resolving (ex: no internet
connection) would make Kong use an invalid Nginx template (with
variable placeholders untouched).

kong/cmd/utils/prefix_handler.lua

@@ -177,7 +177,7 @@ local function compile_conf(kong_config, conf_template)
   local post_template = pl_template.substitute(conf_template, compile_env)
   return string.gsub(post_template, "(${%b{}})", function(w)
     local name = w:sub(4, -3)
-    return compile_env[name:lower()]
+    return compile_env[name:lower()] or ""
   end)
 end