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Allow JSON null in JSON-able values (#873)

In value-only JSON, an empty element of a SubmodelElementList is
serialized as ``null`` to preserve the list indices (e.g.,
``[10, null]``), but we rejected null in ``JSONValue``. We now include
``None`` in ``JSONValue``, so a null may appear at any depth, including
as the value itself.

An optional ``JSONValue`` holding a null counts as absent, and we omit
the key on serialization in all the targets.

In the meta-model, ``is None`` and ``is not None`` over a non-optional
``JSONValue`` (a guarded index or a required property) now check for
a JSON null:

    m is not None and "k" in m and m["k"] is not None

An unguarded key index still can not be checked for nullness since it
would conflate a missing key with a null.

JSON schema includes ``{"type": "null"}``, and XML-RPC uses
the widespread ``<nil/>`` extension, reflected in the XSD.

In TypeScript, we now tell success from failure in ``Either`` by
the error alone, so that ``null`` can be a successfully parsed value.

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