Bug 1600454 - Fix build with --disable-update-channel. r=glandium

Depends on D55363

Differential Revision: https://phabricator.services.mozilla.com/D55364

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

build/moz.configure/init.configure

@@ -1247,7 +1247,7 @@ option('--enable-update-channel',
 
 @depends('--enable-update-channel')
 def update_channel(channel):
-    if channel[0] == '':
+    if not channel or channel[0] == '':
         return 'default'
     return channel[0].lower()