layer_noise_model should accept None

[jyu00]

Describe the bug

The description for layer_noise_model currently says

    layer_noise_model: Union[UnsetType, NoiseLearnerResult, Sequence[LayerError]] = Unset
    r"""A :class:`NoiseLearnerResult` or a sequence of :class:`LayerError`
        objects. If set, all the mitigation strategies that require noise data (e.g., PEC and
        PEA) skip the noise-learning stage, and instead gather the required information from
        ``layer_noise_model``. Layers whose information is missing in ``layer_noise_model``
        are treated as noiseless and their noise is not mitigated.
    """

The Unset value is used to indicate server default will be used, and that default is subject to change.

This option should accept None as a valid value for a user to explicitly indicate they don't want to use an existing model, nor do they want to use the server default (even if that default is not to use any existing model). Not accepting None also means if I set it by mistake, there is no way to unset it (Unset is not public).

The docstring should also say what the server default is (e.g. None).

cc @SamFerracin

Steps to reproduce

Expected behavior

Suggested solutions

Additional Information

• qiskit-ibm-runtime version:
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