adapter.json: `Entity` serializes an empty `specificAssetIds` array, which the schema rejects

Start in sdk/basyx/aas/adapter/json/json_serialization.py around lines 711-712 and compare Entity with the neighbouring collection guards and AssetInformation at line 320. Run JsonSerializationSchemaTest.test_full_example_serialization and test_aas_example_serialization with the AAS schema available; done means co-managed entities without specific asset IDs pass schema validation without an empty array.

Entity.specific_asset_id is a collection, so it is never None, but the serializer guards it with is not None:

https://github.com/eclipse-basyx/basyx-python-sdk/blob/develop/sdk/basyx/aas/adapter/json/json_serialization.py#L711-L712

An entity without specific asset ids therefore serializes as "specificAssetIds": [], and every array in the AAS JSON schema carries minItems: 1. Since AASd-014 forbids specific asset ids on a co-managed entity, every co-managed Entity this SDK writes is invalid JSON — the example AAS included.

Reproduction against the official schema (aas-specs-metamodel v3.1.2):

import json, jsonschema
from basyx.aas import model
from basyx.aas.adapter.json import AASToJsonEncoder

schema = json.load(open("aas.json"))
entity = model.Entity("MyEntity", model.EntityType.CO_MANAGED_ENTITY)
data = json.loads(json.dumps(entity, cls=AASToJsonEncoder))
# {'idShort': 'MyEntity', 'modelType': 'Entity', 'entityType': 'CoManagedEntity', 'specificAssetIds': []}

jsonschema.validate(data, {"$ref": "#/definitions/Entity", "definitions": schema["definitions"]})
# ValidationError: [] should be non-empty

JsonSerializationSchemaTest.test_full_example_serialization and test_aas_example_serialization both catch this as soon as the schema file is actually present — which it isn't in CI, see the companion issue on the schema path.

The neighbouring attributes in the same method, and AssetInformation at L320, already guard on the collection being non-empty; Entity is the only place that doesn't. I checked the other element types against the schema (Submodel, SubmodelElementCollection, SubmodelElementList, Property, Operation, AnnotatedRelationshipElement, AssetAdministrationShell, ConceptDescription) and they all serialize valid documents.

Happy to send the one-line fix with tests.