Do not encode value with resolver if it is None

[frankenjoe]

Currently, encoding a value that is None will raise an error:

class MyObject(audobject.Object):

    @audobject.init_decorator(
        resolvers={
            'arg': audobject.resolver.Tuple,
        }
    )
    def __init__(
            self,
            arg: typing.Tuple = None,
    ):
        super().__init__()
        self.arg = arg

MyObject().to_yaml_s(include_version=False)

TypeError: 'NoneType' object is not iterable