fix: memtrack artifact emptiness check

[not-matthias]

The zstd compressed file will never be empty on disk, so we need to check the contents.

---

Tested on codspeed-go:

ok      example 9.638s
[INFO  codspeed_go_runner] Results written to "/tmp/profile.ZKnRO854Mi.out/results/117605.json"
[DEBUG codspeed_memtrack] Command exited with status: exit status: 0
[DEBUG codspeed_memtrack] Waiting for the drain thread to finish
[DEBUG codspeed_memtrack] Waiting for the writer thread to finish
[INFO  codspeed_memtrack] Wrote 0 memtrack events to disk
[DEBUG::runner_shared::artifacts] Saved ExecutionTimestamps result to "/tmp/profile.ZKnRO854Mi.out/results"
[DEBUG::codspeed::executor::memory::executor] cmd exit status: ExitStatus(unix_wait_status(0))

►►► Tearing down environment 
Error: No memtrack artifact files found. Does the integration support memory profiling?

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing cod-2064-runner-panics-with-no-memtrack-events-found-when-there-are (5985ba0) with main (622460e)}

Summary

✅ 4 untouched benchmarks