Bug in gmin and blank strings

[khotilov]

min of character with blank strings can give wrong results:

> library(data.table)
> d = data.table(id=c(1,1), a=c("","a"))
> d[, min(a), id]
      id     V1
   <num> <char>
1:     1      a
> data.table:::gmin(c("a", ""))
[1] ""
> data.table:::gmin(c("", "a"))
[1] "a"

Expected behavior:

> d[, base::min(a), id]
      id     V1
   <num> <char>
1:     1
> min(c("a", ""))
[1] ""
> min(c("", "a"))
[1] ""

A possible fix is in #4847 .

Output of sessionInfo()

R version 3.6.3 (2020-02-29) Platform: x86_64-pc-linux-gnu (64-bit) Running under: Ubuntu 20.04.1 LTS

Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0

locale:
[1] LC_CTYPE=C.UTF-8 LC_NUMERIC=C LC_TIME=C.UTF-8
[4] LC_COLLATE=C.UTF-8 LC_MONETARY=C.UTF-8 LC_MESSAGES=C.UTF-8
[7] LC_PAPER=C.UTF-8 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] data.table_1.13.5

loaded via a namespace (and not attached):
[1] compiler_3.6.3