Don’t overwrite existing file if appending an empty data frame

[sbearrows]

Fixes tidyverse/readr#1408

Previously, if vroom_write() was supplied an empty data frame, vroom created an empty file without checking the value of append. But this meant data from the file could be deleted such as in this case.

library(vroom)
tf <- withr::local_tempfile()
data <- tibble::tibble(a = "1", b = "2", c = "3")

vroom_write(data, file = tf)
vroom(tf, show_col_types = FALSE)
#> # A tibble: 1 × 3
#>       a     b     c
#>   <dbl> <dbl> <dbl>
#> 1     1     2     3

vroom_write(data.frame(), file = tf, append = TRUE)
vroom(tf, show_col_types = FALSE)
#> # A tibble: 0 × 0

^{Created on 2022-07-15 by the reprex package (v2.0.1.9000)}

[jennybc]

I recommend turning off altrep / lazy reading in this test.

[sbearrows]

@jennybc I believe this is ready for another look!