gracefully handle lack of new files (in quidel indicator)

[melange396]

this line throws an exception when df_list is empty:

covidcast-indicators/quidel_covidtest/delphi_quidel_covidtest/pull.py

Line 78 in 833e818

return pd.concat(df_list), time_flag

instead of crashing, produce a more informative log message.

examples in production: https://logs-prod-01.delphi.cmu.edu/app/r/s/1OZlM