Should rbindlist(..., fill=TRUE) return NA_logical_ in list columns? #4198
Description
From #4196:
When filling a list column, rbindlist departs from the behaviour of all other column types, and returns NULL elements instead of NA:
> A = data.table(c1=0, c2=list(1:3))
> B = data.table(c1=1)
> rbind(A,B,fill=TRUE)
c1 c2
<num> <list>
1: 0 1,2,3
2: 1
Expected:
> A = data.table(c1=0, c2=list(1:3))
> B = data.table(c1=1)
> rbind(A,B,fill=TRUE)
c1 c2
<num> <list>
1: 0 1,2,3
2: 1 NA
Should we change this behaviour for list columns to fill the rows with NA values to match the behaviour of fill=TRUE for other column types?