fix: Fix CachedContent.create if the kms_key_name is not given

PiperOrigin-RevId: 764978933

Author: vertex-sdk-bot

vertexai/caching/_caching.py

@@ -105,7 +105,9 @@ def _prepare_create_request(
  expire_time=expire_time,
  ttl=ttl,
  display_name=display_name,
- encryption_spec=EncryptionSpec(kms_key_name=kms_key_name),
+ encryption_spec=EncryptionSpec(kms_key_name=kms_key_name)
+ if kms_key_name
+ else None,
  ),
  )
  serialized_message_v1beta1 = type(request_v1beta1).serialize(request_v1beta1)