[VisualEditor], [DonationInterface-WorldPayGateway] Ignore empty
messages

https://gerrit.wikimedia.org/r/#/c/169812/1/worldpay_gateway/i18n/en.json,unified
https://gerrit.wikimedia.org/r/#/c/169846/2/modules/ve-mw/i18n/en.json,unified

Change-Id: Ib07aa91fa46a4dc40cd119feb62fdc8a99346cd1