growth_rate() gives hard error in corner cases

[dajmcdon]

The problem seems to be "constant zero" which should probably just be caught. Suggest we return the same result as with all 1's.

library(epiprocess)
#> Loading required package: epidatasets
#> Registered S3 method overwritten by 'tsibble':
#>   method               from 
#>   as_tibble.grouped_df dplyr
#> 
#> Attaching package: 'epiprocess'
#> The following object is masked from 'package:stats':
#> 
#>     filter

am_samoa <- covid_case_death_rates %>% 
  filter(geo_value == "as")
growth_rate(am_samoa$death_rate)
#> Error in stats::approx(x, g, x0): need at least two non-NA values to interpolate

am_samoa
#> An `epi_df` object, 366 x 4 with metadata:
#> * geo_type  = state
#> * time_type = day
#> * as_of     = 2023-03-10
#> 
#> # A tibble: 366 × 4
#>    geo_value time_value case_rate death_rate
#>  * <chr>     <date>         <dbl>      <dbl>
#>  1 as        2020-12-31         0          0
#>  2 as        2021-01-01         0          0
#>  3 as        2021-01-02         0          0
#>  4 as        2021-01-03         0          0
#>  5 as        2021-01-04         0          0
#>  6 as        2021-01-05         0          0
#>  7 as        2021-01-06         0          0
#>  8 as        2021-01-07         0          0
#>  9 as        2021-01-08         0          0
#> 10 as        2021-01-09         0          0
#> # ℹ 356 more rows
all(am_samoa$death_rate == 0)
#> [1] TRUE

# works
growth_rate(rep(1, 100))
#>   [1]  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
#>  [26]  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
#>  [51]  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
#>  [76]  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0 NA

^{Created on 2025-02-11 with reprex v2.1.1}