Fix OptionSet variant JSON encoding for empty arrays and scalars

[LivingLikeKrillin]

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The OPTION_SET branch in OpcUaJsonEncoder.encodeVariantValue derived the JSON typeId by calling Array.get(value, 0) on the variant value. This failed in two real cases: empty arrays threw ArrayIndexOutOfBoundsException, and single (non-array) OptionSet values threw IllegalArgumentException("Argument is not an array"). The latter is reachable via the existing scalar pattern in production code, e.g. new Variant(defaultAccessRestrictions) in NamespaceMetadataTypeNode.

Derive the typeId from the value's class instead, using the existing OptionSetUI8/UI16/UI32/UI64 subclasses — the approach the original TODO comment in this branch proposed. OptionSetUI64 is supported preemptively even though no DataType in stack-core currently extends it.

Test coverage in OpcUaJsonEncoderTest.encodeVariantOptionSet covers the width × cardinality matrix for the OptionSet subclasses in current use: scalar UI8, non-empty UI8/UI16/UI32, and empty UI8/UI16/UI32.

[kevinherron]

Thank you for contributing.

I've already fixed this as part of a larger batch of JSON encoding fixes, and your PR just reminded me that I need to push/PR/merge them :)

See #1758

So I will close this, but thank you anyway.