Set correct defaults for Binaries metadata field

fdroidserver/metadata.py

@@ -57,7 +57,7 @@ app_defaults = OrderedDict([
     ('Requires Root', False),
     ('Repo Type', ''),
     ('Repo', ''),
-    ('Binaries', ''),
+    ('Binaries', None),
     ('Maintainer Notes', []),
     ('Archive Policy', None),
     ('Auto Update Mode', 'None'),

fdroidserver/publish.py

@@ -119,7 +119,7 @@ def main():
             sys.exit(1)
         app = allapps[appid]
 
-        if 'Binaries' in app:
+        if app.get('Binaries', None):
 
             # It's an app where we build from source, and verify the apk
             # contents against a developer's binary, and then publish their