[fix] Set empty app argument value only when it's None

src/yunohost/app.py

@@ -1537,7 +1537,7 @@ def _parse_args_from_manifest(manifest, action, args={}, auth=None):
                     and not arg.get('optional', False):
                 raise MoulinetteError(errno.EINVAL,
                     m18n.n('app_argument_required', name=arg_name))
-            elif not arg_value:
+            elif arg_value is None:
                 args_dict[arg_name] = ''
                 continue