covidcast_signal and empty results

[sgratzl]

in www-main: https://github.com/cmu-delphi/www-main/runs/2228785215?check_suite_focus=true

we currently have the problem that the build fails since it tries to select a field time_value that doesn't exist when covidcast_signal returns an empty set (due to bigint issue)

query_date <- "2020-08-01"
query_date <- "2020-08-01"
covidcast_signal(
  data_source = "quidel",
  signal = "covid_ag_raw_pct_positive",
  start_day = query_date,
  end_day = query_date,
  geo_type = "state",
  geo_value = "ny",
  issues = c(query_date, "2020-09-04")) %>%
    select(time_value, value, sample_size, issue, lag) %>%
    distinct(value, .keep_all=TRUE) %>%
    knitr::kable("html", digits = 2,
                 col.names = c("Test date", "Positivity rate (%)", "Sample size",
                               "Issued on", "Lag (days)"))

imo even if covidcast_signal doesn't return any result it should still have the common structure but no rows