Allow optional first/second in relationship elements

[JAB1305]

Metamodel v3.1.2 changed RelationshipElement.first and second to be optional. This was missed earlier and required by JSON and XML adapters.

• AnnotatedRelationshipElement.__init__ now accepts first and second as Optional[Reference], matching the parent class.
• The JSON and XML deserializers treat both fields as optional default to None when absent.
• The JSON serializer omits the fields when None (XML serializer already handled this).
• Added tests for this behaviour.

Fixes #525

[s-heppner]

LGTM, does the xml_serialization not need any change?

[JAB1305]

LGTM, does the xml_serialization not need any change?

No, I only adjusted JSON serialization to have it behave the same as its XML counterpart. The XML serialization does not emit the fields if None:

basyx-python-sdk/sdk/basyx/aas/adapter/xml/xml_serialization.py

Lines 729 to 743 in 06a9b62

	def relationship_element_to_xml(obj: model.RelationshipElement,
	tag: str = NS_AAS+"relationshipElement") -> etree._Element:
	"""
	Serialization of objects of class :class:`~basyx.aas.model.submodel.RelationshipElement` to XML
	:param obj: Object of class :class:`~basyx.aas.model.submodel.RelationshipElement`
	:param tag: Namespace+Tag of the serialized element (optional). Default is ``aas:relationshipElement``
	:return: Serialized :class:`~lxml.etree._Element` object
	"""
	et_relationship_element = abstract_classes_to_xml(tag, obj)
	if obj.first is not None:
	et_relationship_element.append(reference_to_xml(obj.first, NS_AAS+"first"))
	if obj.second is not None:
	et_relationship_element.append(reference_to_xml(obj.second, NS_AAS+"second"))
	return et_relationship_element

Previously JSON serialization did add the fields.