this is a useful test case, but please add a few more cases on this theme covering tuples with `nan` and various pandas structures. These particularly look at how heterogeneous data structures should work.

```
1. isnull([(False,), True])
2. isnull(np.array([(1,), 0)])
3. isnull([(np.nan,), 5.0])
4. isnull(np.array([5.0, np.nan, (np.nan,)], dtype=float))
5. ser = Series(np.array([5.0, np.nan, (np.nan,)], dtype=float); isnull(ser); ser.isnull() [also should test SparseSeries here]
6. ser = Series([(1,), (5,), (6, 8, np.nan), (np.nan,)]); isnull(ser); ser.isnull() [+ SparseSeries]
7. df = DataFrame([[(1,), (5,), (6, 8, np.nan), (np.nan,)]]); isnull(df); df.isnull() [+ SparseDataFrame]
7. df=DataFrame([[(np.nan,), 5.0, np.nan]]); df.isnull(); isnull(df) [also test SparseDataFrame here]
```