fix: correct None check for query_time in xlsx response_time_s output

[mango766]

Fixes #2891

The --xlsx output block initializes response_time_s as a list and then checks if response_time_s is None before appending — but a list is never None, so the fallback empty string path is dead code. Sites that don't return a response time (timeouts, errors) end up with raw None values in the Excel column instead of a blank cell.

The fix mirrors what the --csv block already does correctly: check results[site]["status"].query_time is None rather than the accumulator list.

One-line change, no behaviour change for sites that do return a response time.