Handle empty Android benchmark results (#5916)

Summary:
The script didn't handle invalid JSON well when the benchmark results is empty https://github.com/pytorch/executorch/actions/runs/11197448597/job/31128464106.  It's better to upload whatever it finds instead of crashing in this case.

Looking a bit close at the benchmark jobs, there are some cases where the `benchmark_results.json` file is empty.  I'm not sure why yet, but it can be fixed in another PR:

* https://github.com/pytorch/executorch/actions/runs/11197448597/job/31127998979#step:15:286 (stories110M, qnn)
* https://github.com/pytorch/executorch/actions/runs/11197448597/job/31127999221#step:15:946 (vit, xnnpack)

The test spec already waits for 3 minutes to see if the file is there before giving up

Pull Request resolved: https://github.com/pytorch/executorch/pull/5916

Reviewed By: guangy10

Differential Revision: D63950793

Pulled By: huydhn

fbshipit-source-id: 160d1465395c025e028b0e2cb69b5837a8f0208a