Bug fixes + removal of compiler warnings

> Fixed compiler warnings in 'rbindlist' from eddi's commit in 1012.
> Fixed a potential segfault from the last commit (1098) for the case rbindlist(list())
> Fixed #5300 - rbind(DT, NULL) doesn't result in error anymore (no NEWS added as it got changed during 1012 - not a new feature)
> Modified test 680 to reflect eddi's changes in commit 1012 (added sufficient notes).