in a dataframe `df` how can I find the columns that contains all nan after grouping the rows?

```
In [97]: df
Out[97]:
     a    b  group
0  NaN  NaN  a
1  0.0  NaN  a
2  2.0  NaN  a
3  1.0  7.0  b
4  1.0  3.0  b
5  7.0  4.0  b
6  2.0  6.0  c
7  9.0  6.0  c
8  3.0  0.0  c
9  9.0  0.0  c

```

in this case the desired output should be 
group: a - columns: b