Fix the script defaults

fdroidserver/metadata.py

@@ -78,7 +78,7 @@ flag_defaults = OrderedDict([
     ('commit', None),
     ('subdir', None),
     ('submodules', False),
-    ('init', None),
+    ('init', ''),
     ('patch', []),
     ('gradle', False),
     ('maven', False),
@@ -91,12 +91,12 @@ flag_defaults = OrderedDict([
     ('forcevercode', False),
     ('rm', []),
     ('extlibs', []),
-    ('prebuild', []),
+    ('prebuild', ''),
     ('update', ['auto']),
     ('target', None),
     ('scanignore', []),
     ('scandelete', []),
-    ('build', []),
+    ('build', ''),
     ('buildjni', []),
     ('preassemble', []),
     ('antcommand', None),