Clarify error message on by = list()

[MichaelChirico]

This line has an error message:

https://github.com/Rdatatable/data.table/blob/master/R/data.table.R#L954

if (length(bysubl)<2L) stop("When 'by' or 'keyby' is list() we expect something inside the brackets")

However I don't think it's possible to reach that branch in the current code flow because:

DT = data.table(a = 1:10)
DT[ , sum(a), by = .()]
#    V1
# 1: 55

This output looks OK to me... should we just axe this error, or should we move the branch logic so that it actually gets triggered (e.g. put the length(bysubl) < 2L condition a few lines earlier)?

[jangorecki]

Mentioned error has been already removed, I filled PR adding unit tests for current behaviour. It will also address my issue reported in #648.