code: def test_metrics_diff_no_metrics(tmp_dir, scm, dvc):
      tmp_dir.scm_gen({"foo": "foo"}, commit="add foo")
      assert dvc.metrics.diff(a_rev="HEAD~1") == {}
  
imports: import json
  from os.path import join
  import pytest
  from dvc.cli import main
  from dvc.utils import relpath
  from dvc.utils.serialize import JSONFileCorruptedError
label_desc: Use image node_id 6 for calling a global function with example usage: test_metrics_diff_no_metrics(tmp_dir, scm, dvc) without return types