Fixes for #5377, #5405, forder edge case, data.table(.) edge cases

+ data.table(list()), data.table(data.table()), data.table(data.frame()) return null data.table as expected. Fix for #5377.
+ forder edge case - forder(length 0 VECSXP) now returns integer(0) to be consistent with base:::order.
+ Minor change to test.data.table so that cc(TRUE) runs smoothly on mac. Hopefully it doesn't break any of yours. If so, please let me know.
+ Fix for #5405. unique(.) on null data.table returns null data.table now. Added test 1224.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1206 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
NEWS/comments only.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1207 edb9625f-4e0d-4859-8d74-9fd3b1da38cb