Bug 1614518 - Followup fix to undo unneeded part of change that breaks tests

Differential Revision: https://phabricator.services.mozilla.com/D63533

--HG--
extra : moz-landing-system : lando

python/mach/mach/decorators.py

@@ -97,7 +97,7 @@ class _MachCommand(object):
             raise ValueError('can only operate on _MachCommand instances')
 
         for a in self.__slots__:
-            if getattr(self, a) is None:
+            if not getattr(self, a):
                 setattr(self, a, getattr(other, a))
 
         return self