The various *FLAGS variables are defined as nargs=1, which guarantees
that if they are set they are going to have a value. They also come with
a default value of "", which works just fine in the default case.
Things change, though, when the variable is explicitly set to an empty
value, which then makes it a NegativeOptionValue, which is different
from the default and needs to be handled.
Differential Revision: https://phabricator.services.mozilla.com/D217328
1c798c12cb