consistent treatment of nested lists in replacement

[tdhock]

since #6167 we have

> DT1=data.table(L=list("A"),i=1)
> DT1$L=list(list(list(NULL)));str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
 $ L:List of 1
  ..$ :List of 1
  .. ..$ : NULL
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 
> DT1$L=list(list(NULL));str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
 $ L:List of 1
  ..$ : NULL
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 
> DT1$L=list(NULL);str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
 $ L:List of 1
  ..$ : NULL
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 
> DT1$L=NULL;str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  1 variable:
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 

interpretation:

• three nested list with NULL in code gives two nested lists in data.
• one and two nested lists with NULL in code gives one list in data.

this is a weird edge case, that probably nobody will ever use, but it may be surprising to some users that list(list(NULL)) and list(NULL) give the same result, I would have expected that list(list(NULL)) results in two nested lists in the data, but to get that you actually have to use three lists in the code.

for reference, before #6167 we have similar behavior:

> DT1=data.table(L=list("A"),i=1)
> DT1$L=list(list(list(NULL)));str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
 $ L:List of 1
  ..$ :List of 1
  .. ..$ : NULL
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 
> DT1$L=list(list(NULL));str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
 $ L:List of 1
  ..$ : NULL
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 
> DT1$L=list(NULL);str(DT1)
Classes ‘data.table’ and 'data.frame':	1 obs. of  1 variable:
 $ i: num 1
 - attr(*, ".internal.selfref")=<externalptr> 

summary table

RHS	before #6167	after #6167
list(NULL)	delete column	list(NULL)
list(list(NULL))	list(NULL)	list(NULL)
list(list(list(NULL)))	list(list(NULL))	list(list(NULL))

so actually I'm not sure if we should even do anything about this... what do you think @MichaelChirico @joshhwuu ?
feel free to close if you think it is too useless / obscure / annoying.... haha.