[Wikibase] Add 1 more message as optional

Spotted by Shirayuki
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-ui-pendingquantitycounter-pending-pendingsubpart/en

Change-Id: I6f6095dcfcfd3cb1964a5ffca8eb5f800fb9510e
Merge "[Wikibase] Add 1 more message as optional"