Fixing None handling error for legacy_export

BUG=None
TEST=None

Change-Id: I69f249db73b103f13fcae23fd7ff4292cce9d482
Reviewed-on: https://gerrit.chromium.org/gerrit/37316
Reviewed-by: Jay Kim <yongjaek@chromium.org>
Tested-by: Jay Kim <yongjaek@chromium.org>
diff --git a/hwid_tool.py b/hwid_tool.py
index 7458706..041c7f4 100755
--- a/hwid_tool.py
+++ b/hwid_tool.py
@@ -964,7 +964,8 @@
       export_data['version_' + ic_class] = [ic_value]
     export_data['config_factory_initial'] = sorted(
         'version_' + ic_class for ic_class in initial_config)
-    export_data.update(extra_fields)
+    if extra_fields != None:
+        export_data.update(extra_fields)
     hwid_file_name = ('components ' + hwid_str).replace(' ', '_')
     hwid_file_path = os.path.join(config.dest_dir, hwid_file_name)
     with open(hwid_file_path, 'w') as f: