`fread()` does not report `""` as `NA` even when specified in `na.strings`

[AngelFelizR]

Description

When reading a CSV file that contains empty quoted strings (""), I expect these values to be parsed as NA_character_ if explicitly defined in the na.strings argument. However, fread() does not display the NAs accordingly. Instead, the fields are shown as empty strings in the output.

Minimal reproducible example

temp_file <- tempfile(fileext = ".csv")

writeLines(
  text = c(
    '"Date Example","Question 1","Question 2","Site: Country"',
    '"12/5/2012","Yes","Yes","Chile"',
    '"","","","Virgin Islands, British"'
  ),
  con = temp_file
)

# Missing values not shown as NA by default
data.table::fread(temp_file)
#>    Date Example Question 1 Question 2           Site: Country
#>          <char>     <char>     <char>                  <char>
#> 1:    12/5/2012        Yes        Yes                   Chile
#> 2:                                    Virgin Islands, British

# Missing values still not shown as NA, even with na.strings = '""'
data.table::fread(temp_file, na.strings = '""')
#>    Date Example Question 1 Question 2           Site: Country
#>          <char>     <char>     <char>                  <char>
#> 1:    12/5/2012        Yes        Yes                   Chile
#> 2:                                    Virgin Islands, British

unlink(temp_file)

sessionInfo()
#> R version 4.2.3 (2023-03-15 ucrt)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 26100)
#> 
#> Matrix products: default
#> 
#> locale:
#> [1] LC_COLLATE=English_United States.utf8 
#> [2] LC_CTYPE=English_United States.utf8   
#> [3] LC_MONETARY=English_United States.utf8
#> [4] LC_NUMERIC=C                          
#> [5] LC_TIME=English_United States.utf8    
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> loaded via a namespace (and not attached):
#>  [1] digest_0.6.31      withr_2.5.2        R.methodsS3_1.8.2  lifecycle_1.0.4   
#>  [5] magrittr_2.0.3     reprex_2.0.2       evaluate_0.23      rlang_1.1.3       
#>  [9] cli_3.6.2          data.table_1.17.99 rstudioapi_0.15.0  fs_1.6.3          
#> [13] R.utils_2.12.2     R.oo_1.25.0        vctrs_0.6.5        styler_1.10.2     
#> [17] rmarkdown_2.25     tools_4.2.3        R.cache_0.16.0     glue_1.7.0        
#> [21] purrr_1.0.1        xfun_0.39          yaml_2.3.10        fastmap_1.1.1     
#> [25] compiler_4.2.3     htmltools_0.5.5    knitr_1.45

^{Created on 2025-05-13 with reprex v2.0.2}

Expected behavior

I expect the second row’s empty quoted fields ("") to be shown as NA values when na.strings = '""' is provided. Instead, they are displayed as empty strings.

Thank you for your great work on data.table!