Fix initializer load for in-memory external data (#29349)
### Description
The `Initializer(graph, …)` constructor wrongly required a `model_path`
for an initializer with in-memory external data when its `OrtValue`
wasn't registered. It now loads the data from memory via
`CreateTensorFromTensorProto`, matching the other constructor.
### Motivation and Context
Fixes #28267. A model loaded from bytes via `CreateSessionFromArray`
failed with `model_path must not be empty`; loading the same model from
a file worked.
