private_message-3486047/3486047-do-not-show: Added the functionality to...

private_message-3486047/3486047-do-not-show: Added the functionality to show/hide when new messages or threads are zero.

Closes #3486047

config/schema/private_message.schema.yml

@@ -141,3 +141,6 @@ block.settings.private_message_notification_block:
     count_method:
       type: string
       label: 'Return of count messages or threads'
+    hide_zero_count:
+      type: boolean
+      label: 'Hide zero count'