Fix linter

Author: RReverser

test/common.py

@@ -1697,7 +1697,7 @@ def assertBinaryEqual(self, file1, file2):
     self.assertEqual(read_binary(file1),
                      read_binary(file2))
 
-  def checkOutputSizes(self, outputs: List[str], metadata={}):
+  def checkOutputSizes(self, outputs: List[str], metadata=None):
     test_name = self.id().split('.')[-1]
     results_file = test_file('code_size', test_name + '.json')
 
@@ -1754,7 +1754,8 @@ def print_diff(title, actual, expected):
     print_diff('Total output size', total_output_size, total_expected_size)
     print_diff('Total output size gzipped', total_output_size_gz, total_expected_size_gz)
 
-    obtained_results.update(metadata)
+    if metadata is not None:
+      obtained_results.update(metadata)
 
     if EMTEST_REBASELINE:
       create_file(results_file, json.dumps(obtained_results, indent=2) + '\n', absolute=True)