As suggested in the review for bug 1793005 (D158441). Sets gAppData->vendor to
NULL when otherwise falsy without changing the value set by old-configure. There
are places in the code such as BackgroundTasks.cpp from toolkit that depend on
MOZ_APP_VENDOR being a string value, empty or otherwise.

Differential Revision: https://phabricator.services.mozilla.com/D160651