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

[jreback]

 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

related #5720, #5744

going back to 0.12 behavior
effectively can add a column by assigning a scalar to a frame that doesn't have an index
need a more compelling reason to raise here

In [4]: df = DataFrame({"A": [1, 2, 3], "B": [1.2, 4.2, 5.2]})

In [5]: y = df[df.A > 5]

In [6]: y
Out[6]: 
Empty DataFrame
Columns: [A, B]
Index: []

[0 rows x 2 columns]

In [7]: y['New'] = np.nan

In [8]: y
Out[8]: 
Empty DataFrame
Columns: [A, B, New]
Index: []

[0 rows x 3 columns]