onnxruntime

Refine error checks in onnxruntime/core/providers/coreml/model/model.mm. (#18620)

#18606 updated the original error checks to check that the returned object != nil to appease the static analyzer. However, per the API docs, checking `error != nil` is the way to determine whether an error occurred. This change adds back the `error != nil` check to be safe.