[SYCL][GRAPH] Add empty method on graph (#21554)
## Summary
Adds `empty()` on modifiable command graphs so callers can test graph
emptiness without relying on `get_nodes()`.
## Motivation
We need to support **both native Level Zero graphs and SYCL graphs**
behind a common usage pattern. Native L0 graph APIs expose something
like **`empty()`** but **not** **`get_nodes()`**.
**PyTorch** (and similar stacks) today often checks emptiness via
**`get_nodes()`**. That breaks when switching to **native L0 graphs**,
where `get_nodes()` is not available. Exposing **`empty()`** on the SYCL
modifiable graph matches what native graphs can offer and gives PyTorch
a single, portable way to ask “is this graph empty?” for both paths.
## Changes
- **`modifiable_command_graph::empty()`** — `true` when the graph has no
nodes.
- **Spec** — `empty()` documented in `sycl_ext_oneapi_graph.asciidoc`
(aligned with `get_nodes` / `get_root_nodes`).
- **API docs** — Doxygen on `empty()` in `modifiable_graph.hpp`.
- **Tests** — `CommandGraphTest::GraphEmpty` in `Queries.cpp` (public
API, consistency with `get_nodes()`, vs `graph_impl`).
## Testing
- `CommandGraphExtensionTests` — includes new `GraphEmpty` coverage.
