FIX: Admin sidebar problem check missing -1 check (#30916)

When we initially turned on admin sidebar for new sites,
existing sites had the value set to -1. We need to show
the problem check to these sites too, but currently it only
checks if `admin_sidebar_enabled_groups` is empty.

app/services/problem_check/admin_sidebar_deprecation.rb

@@ -4,7 +4,10 @@ class ProblemCheck::AdminSidebarDeprecation < ProblemCheck::ProblemCheck
   self.priority = "low"
 
   def call
-    return no_problem if SiteSetting.admin_sidebar_enabled_groups.present?
+    if SiteSetting.admin_sidebar_enabled_groups.present? &&
+         SiteSetting.admin_sidebar_enabled_groups != "-1"
+      return no_problem
+    end
 
     problem
   end