rbind and rbindlist drop factor levels when data.table has zero rows

[gdemin]

In the previous data.table versions (before 1.12.2) factor levels were preserved during rbind even if data.table has zero rows. See example:
# Minimal reproducible example

library(data.table)

DT = data.table(f = factor(c("a", "b", "c")))

# all ok, rbind works as expected
str(rbind(DT, DT))
# Classes ‘data.table’ and 'data.frame':	6 obs. of  1 variable:
#     $ f: Factor w/ 3 levels "a","b","c": 1 2 3 1 2 3
# - attr(*, ".internal.selfref")=<externalptr> 
zeroDT = DT[FALSE,]
str(zeroDT)
# Classes ‘data.table’ and 'data.frame':	0 obs. of  1 variable:
#     $ f: Factor w/ 3 levels "a","b","c": 
#     - attr(*, ".internal.selfref")=<externalptr> 

# zero rows data.table
# unexpected 0 factor levels
str(rbind(zeroDT, zeroDT))
# Classes ‘data.table’ and 'data.frame':	0 obs. of  1 variable:
#     $ f: Factor w/ 0 levels: 
#     - attr(*, ".internal.selfref")=<externalptr> 
str(rbindlist(list(zeroDT, zeroDT)))
# Classes ‘data.table’ and 'data.frame':	0 obs. of  1 variable:
#     $ f: Factor w/ 0 levels: 
#     - attr(*, ".internal.selfref")=<externalptr>

# Output of sessionInfo()

R version 3.5.3 (2019-03-11)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Linux Mint 19.1

Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/openblas/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/libopenblasp-r0.2.20.so

locale:
[1] LC_CTYPE=ru_RU.UTF-8       LC_NUMERIC=C               LC_TIME=ru_RU.UTF-8        LC_COLLATE=ru_RU.UTF-8    
[5] LC_MONETARY=ru_RU.UTF-8    LC_MESSAGES=ru_RU.UTF-8    LC_PAPER=ru_RU.UTF-8       LC_NAME=C                 
[9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=ru_RU.UTF-8 LC_IDENTIFICATION=C       

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

other attached packages:
    [1] data.table_1.12.2

loaded via a namespace (and not attached):
    [1] compiler_3.5.3 tools_3.5.3    packrat_0.5.0