os.getxattr should raise OSError(ENODATA) for non-existent extended attributes

[vector-of-bool]

Describe the bug
The faked os.getxattr returns None if the extended attribute does not exist (and is documented as such). The standard os.getxattr either returns bytes or raises an OSError from the underlying syscall. The getxattr syscall fails with ENODATA if the attribute name is legal but is unset, and fails with ENOTSUP if the attribute name is illegal.

How To Reproduce

def read_nonexistent_xattr(s: Path):
    s.write_text("file content")
    # Non-existent attribute:
    with pytest.raises(OSError) as exc:
        os.getxattr(s, "user.nonesuch_attribute")
    assert exc.value.errno == errno.ENODATA
    # Invalid attribute:
    with pytest.raises(OSError) as exc:
        os.getxattr(s, "invalid attribute name")
    assert exc.value.errno == errno.ENOTSUP

def test_realfs():
    read_nonexistent_xattr(Path("file.txt"))

def test_fakefs(fs):
    read_nonexistent_xattr(Path("file.txt"))

Your environment
Please run the following in the environment where the problem happened and
paste the output.

$ python -c "import platform; print(platform.platform())"
Linux-6.7.6-arch1-1-x86_64-with-glibc2.39
$ python -c "import sys; print('Python', sys.version)"
Python 3.11.7 (main, Jan 29 2024, 16:03:57) [GCC 13.2.1 20230801]
$ python -c "from pyfakefs import __version__; print('pyfakefs', __version__)"
pyfakefs 5.4.dev0
$ python -c "import pytest; print('pytest', pytest.__version__)"
pytest 7.2.0

[mrbean-bremen]

Thanks! Keep in mind that real xattr support will not added to pyfakefs, but at least the behavior shall be as expected.