BUG: pd.isnull with tuple argument returns different results compared to list argument
It seems that pandas.isnull returns different results with tuple and list arguments. I would expect to see identical results with tuple argument. 

Here is an example of the issue

``` python
>>> pd.version.version
'0.12.0'
>>> pd.isnull((3,2))
False
>>> pd.isnull([3,2])
array([False, False], dtype=bool)
```
