[torchgen] Small fix for empty yaml file edge case (#92938)
Rely on CI.
Avoid issues such as:
```
Traceback (most recent call last):
File "<string>", line 38, in <module>
File "<string>", line 36, in __run
File "/usr/local/fbcode/platform010/lib/python3.8/runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/local/fbcode/platform010/lib/python3.8/runpy.py", line 87, in _run_code
exec(code, run_globals)
File "/re_cwd/buck-out/v2/gen/fbcode/2841b324ed9b88dd/caffe2/torchgen/__gen_executorch__/gen_executorch#link-tree/torchgen/gen_executorch.py", line 690, in <module>
main()
File "/re_cwd/buck-out/v2/gen/fbcode/2841b324ed9b88dd/caffe2/torchgen/__gen_executorch__/gen_executorch#link-tree/torchgen/gen_executorch.py", line 626, in main
parsed_yaml, custom_ops_parsed_yaml = parse_yaml_files(
File "/re_cwd/buck-out/v2/gen/fbcode/2841b324ed9b88dd/caffe2/torchgen/__gen_executorch__/gen_executorch#link-tree/torchgen/gen_executorch.py", line 505, in parse_yaml_files
translate_native_yaml(
File "/re_cwd/buck-out/v2/gen/fbcode/2841b324ed9b88dd/caffe2/torchgen/__gen_executorch__/gen_executorch#link-tree/torchgen/gen_executorch.py", line 448, in translate_native_yaml
for e in native_es:
TypeError: 'NoneType' object is not iterable
```
Differential Revision: [D42729435](https://our.internmc.facebook.com/intern/diff/D42729435)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92938
Approved by: https://github.com/JacobSzwejbka
