I would like to drop the `[]` for a given `df`

```
df=pd.DataFrame(dict(a=[1,2,4,[],5]))

```

Such that the expected output will be

```
   a
0  1
1  2
2  4
3  5

```

Edit:

or to make thing more interesting, what if we have two columns and some of the cell is with `[]` to be dropped.

```
df=pd.DataFrame(dict(a=[1,2,4,[],5],b=[2,[],1,[],6]))

```