[SYCL][GRAPH] Add empty method on graph

[lslusarczyk]

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.