feat: Negative Tests

[caldempsey]

Adds a few negative tests. These two are suspect:

def test_to_from_base62_invalid_base62_string():
    # Arrange
    ksuid = Ksuid()
    invalid_base62 = "invalid_base62_string!"

    # Act & Assert
    with pytest.raises(ValueError):
        ksuid.from_base62(invalid_base62)

def test_to_from_base62_empty_string():
    # Arrange
    ksuid = Ksuid()
    empty_base62 = ""

    # Act & Assert
    with pytest.raises(IndexError): # this looks wrong
        ksuid.from_base62(empty_base62)