Compatibility of tests with Python 3.15

[frenzymadness]

What pipdeptree version are you using?

2.30.0

Are you running pipdeptree in a virtual environment?

No

Describe the problem

When running tests with Python 3.15 during build of RPM package in Fedora, we get:

=================================== FAILURES ===================================
____________________________ test_invalid_metadata _____________________________

mocker = <pytest_mock.plugin.MockerFixture object at 0x7f18e425f450>
capfd = <_pytest.capture.CaptureFixture object at 0x7f18e4186050>
fake_dist_with_invalid_metadata = PosixPath('/tmp/pytest-of-mockbuild/pytest-0/test_invalid_metadata0/site-packages/bar-2.4.5.dist-info')

    def test_invalid_metadata(
        mocker: MockerFixture, capfd: pytest.CaptureFixture[str], fake_dist_with_invalid_metadata: Path
    ) -> None:
        fake_site_dir = str(fake_dist_with_invalid_metadata.parent)
        mocked_sys_path = [fake_site_dir]
        mocker.patch("pipdeptree._discovery.sys.path", mocked_sys_path)
    
>       dists = get_installed_distributions()
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/test_discovery.py:165: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../BUILDROOT/usr/lib/python3.15/site-packages/pipdeptree/_discovery.py:49: in get_installed_distributions
    return filter_valid_distributions(distributions(path=computed_paths))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../BUILDROOT/usr/lib/python3.15/site-packages/pipdeptree/_discovery.py:89: in filter_valid_distributions
    if not has_valid_metadata(dist):
           ^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

dist = <importlib.metadata.PathDistribution object at 0x7f18e41ec640>

    def has_valid_metadata(dist: Distribution) -> bool:
>       return "Name" in dist.metadata
               ^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: argument of type 'NoneType' is not a container or iterable

../BUILDROOT/usr/lib/python3.15/site-packages/pipdeptree/_discovery.py:119: TypeError
=========================== short test summary info ============================
FAILED tests/test_discovery.py::test_invalid_metadata - TypeError: argument of type 'NoneType' is not a container or iterable
================= 1 failed, 143 passed, 7 deselected in 1.10s ==================

[hroncok]

This looks like python/cpython#143387 to me. dist.metadata is None instead of an empty dict.

[kemzeb]

Reading the backport changelog for v8.7.0, metadata() and the metadata property now return None if the METADATA file is empty or missing instead of returning an empty PackageMetadata. Because of this, I view this as more of a breaking change since we see a change in how missing metadata is returned.

Given the upstream issue opened by @hroncok, it is currently unknown what decision the Python maintainers will make so I'll keep this issue open to make sure we can track their decision and reflect it here.