Currently, if MF definitions are missing (typically because there’s a
compilation error), `I18n.messageFormat` will try to access
`I18n._mfMessages.hasMessage` resulting in a crash that will in turn
crash Ember.
This patch addresses the issue by using the optional chaining operator
making the `I18n.messageFormat` method return a "Missing Key" message.
MF strings won’t be rendered properly, but the site will stay usable.