DEV: Add debug lines for mystery no_user_selected error (#8759)

On some customer forums we are randomly getting a "You must select a valid user" error when sending a PM even when all parameters seem to be OK. This is an attempt to track it down with more data.

app/controllers/posts_controller.rb

@@ -777,6 +777,10 @@ class PostsController < ApplicationController
       result[:target_group_names] = groups.join(",")
     end
 
+    if recipients.blank? || result[:target_usernames].blank?
+      Rails.logger.debug("Missing recipients for PM! result: #{result.inspect} | params: #{params.inspect}")
+    end
+
     result.permit!
     result.to_h
   end