Ambiguous error when using fetch_tbl on a null response

[dshemetov]

A call that produces a null result, such as

y <- covidcast( 
      data_source = "bad-source", 
      signals = "bad-signal", 
      geo_type = "state",  
      time_type = "day", 
      time_value = epirange(20200601, 20221201),
      geo_values = "ca,fl")

produces a cryptic error when requesting fetch_tbl

r$> y %>% fetch_tbl
Error: '' does not exist in current working directory ('/home/dskel/Documents/Code/Delphi/epiprocess').

This is because fetch_tbl calls fetch_csv and attempts to readr::read_csv on the output without validation and, in this case, fetch_csv returns an empty string.