add test for na.omit(, cols=empty) (#4360)
add test for na.omit(, cols=empty) (#4360)


Rdatatable/data.table,jangorecki/data.table,vlulla/data.table,Rdatatable/data.table,jangorecki/data.table,Rdatatable/data.table,jangorecki/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table,Rdatatable/data.table,vlulla/data.table