[tests] When test_venv.test_with_pip() fails, a bug in error handling fails to display stderr #97764

ERROR: test_with_pip (test.test_venv.EnsurePipTest.test_with_pip)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 660, in nicer_error
    yield
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 595, in do_test_with_pip
    self.run_with_capture(venv.create, self.env_dir,
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 87, in run_with_capture
    func(*args, **kwargs)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/venv/__init__.py", line 459, in create
    builder.create(env_dir)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/venv/__init__.py", line 76, in create
    self._setup_pip(context)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/venv/__init__.py", line 349, in _setup_pip
    subprocess.check_output(cmd, stderr=subprocess.STDOUT)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/subprocess.py", line 465, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/subprocess.py", line 569, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/tmp/test_python_92rnuaxl/tmpkaiesc24/bin/python', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 672, in test_with_pip
    self.do_test_with_pip(False)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 594, in do_test_with_pip
    with self.nicer_error():
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/contextlib.py", line 155, in __exit__
    self.gen.throw(value)
  File "/buildbot/buildarea/3.x.pablogsal-arch-x86_64.clang-ubsan/build/Lib/test/test_venv.py", line 663, in nicer_error
    err = exc.stderr.decode(errors="replace")
          ^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'decode'