pd.concat fails when dataframe is empty

[anupambhatnagar]

🐛 Describe the bug

Fails on /tests/data/trace_compare/base

https://github.com/facebookresearch/HolisticTraceAnalysis/blob/main/hta/analyzers/trace_counters.py#L65

Steps to reproduce

Run get_queue_length_time_series for the trace above

Expected behavior

fail gracefully with a warning

Environment

HTA 0.1.2

Additional Info

No response