Fix incorrectly keeping outdated update notices absent from the API endpoint (#27021)

app/services/software_update_check_service.rb

@@ -35,11 +35,13 @@ class SoftwareUpdateCheckService < BaseService
   end
 
   def process_update_notices!(update_notices)
-    return if update_notices.blank? || update_notices['updatesAvailable'].blank?
+    return if update_notices.blank? || update_notices['updatesAvailable'].nil?
 
     # Clear notices that are not listed by the update server anymore
     SoftwareUpdate.where.not(version: update_notices['updatesAvailable'].pluck('version')).delete_all
 
+    return if update_notices['updatesAvailable'].blank?
+
     # Check if any of the notices is new, and issue notifications
     known_versions = SoftwareUpdate.where(version: update_notices['updatesAvailable'].pluck('version')).pluck(:version)
     new_update_notices = update_notices['updatesAvailable'].filter { |notice| known_versions.exclude?(notice['version']) }