`flatline_forecaster` error when using `target_date`

[dshemetov]

TL;DR: target_date results in NA forecasts? If you comment out target_date and uncomment ahead lines below, the forecasts are reasonable.

r$> library(epidatr) # Access Delphi API
    library(epipredict)
    library(dplyr)

    format_storage <- function(pred, true_forecast_date, target_end_date) {
      pred %>%
        mutate(
          forecast_date = true_forecast_date,
          .dstn = nested_quantiles(.pred_distn)
        ) %>%
        unnest(.dstn) %>%
        select(-any_of(c(".pred_distn", ".pred", "time_value"))) %>%
        rename(quantile = quantile_levels, value = values, target_end_date = 
    target_date) %>%
        relocate(geo_value, forecast_date, target_end_date, quantile, value)
    }

    epidata <- pub_covidcast(
      source = "jhu-csse",
      signals = "deaths_incidence_num",
      time_type = "day",
      geo_type = "state",
      geo_values = c("ca", "tx", "fl"),
      time_values = epirange(20210101, 20211231)
    ) %>%
      select(geo_value, time_value, deaths = value) %>%
      as_epi_df()

    fit <- flatline_forecaster(
      epidata,
      outcome = "deaths",
      args_list = flatline_args_list(
        # ahead = 1L,
        n_training = 7L,
        forecast_date = as.Date("2021-12-31"),
        target_date = as.Date("2022-01-01"),
        quantile_levels = c(0.2, 0.5, 0.8),
      )
    )
    format_storage(fit$predictions, as.Date("2021-12-31"), as.Date("2022-01-0
    1"))
# A tibble: 9 × 5
  geo_value forecast_date target_end_date quantile value
  <chr>     <date>        <date>             <dbl> <dbl>
1 ca        2021-12-31    2022-01-08           0.2    NA
2 ca        2021-12-31    2022-01-08           0.5    NA
3 ca        2021-12-31    2022-01-08           0.8    NA
4 fl        2021-12-31    2022-01-08           0.2    NA
5 fl        2021-12-31    2022-01-08           0.5    NA
6 fl        2021-12-31    2022-01-08           0.8    NA
7 tx        2021-12-31    2022-01-08           0.2    NA
8 tx        2021-12-31    2022-01-08           0.5    NA
9 tx        2021-12-31    2022-01-08           0.8    NA