API/REGRESS: partial revert of f8b6208675b5b10d73a74f50478fa5e37b43fc02
     allow assignment of a column in a frame with a scalar with no index (so adds to the columns),
     instead of raising; this preservers 0.12 behavior
Merge pull request #5747 from jreback/empty_frame3

API/REGRESS: partial revert of f8b6208675b5b10d73a74f50478fa5e37b43fc02 (GH5720,GH5744)