python: How can I pass None value in my test part?

How can I get my first statement, when my value is None.

def func(stringg, keyy, valuee):
    values= {r.split('=')[0]: r.split('=')[1] for r in stringg.split(',')}
    if valuee is None:
        del values[keyy]
    else:
        values[keyy] = valuee
    return ','.join([f'{a}={b}' for a, b in values.items()])

This is my function.

@pytest.mark.parametrize('stringg, keyy, valuee, expected_result',
                [('red=4, green=7', 'red', None, 'green=7')])

In this part I'm passing None, but It didn't work.

 def test_func(stringg, keyy, valuee, expected_result):
    result = func(stringg, keyy, valuee)
    assert result == expected_result