Series/DataFrame.isna/notna with sparse data densifies
```
In [1]: df = pd.SparseDataFrame({'A': [np.nan, np.nan, 1, 2, np.nan], 
   ...:                          'B': [0, np.nan, np.nan, 2, np.nan]})                                                                                                          

In [2]: res = df.notna()                                                                                                                                                        

In [3]: res                                                                                                                                                                     
Out[3]: 
       A      B
0  False   True
1  False  False
2   True  False
3   True   True
4  False  False

In [4]: res['A']                                                                                                                                                                
Out[4]: 
0    False
1    False
2     True
3     True
4    False
Name: A, dtype: Sparse[bool, False]
BlockIndex
Block locations: array([2], dtype=int32)
Block lengths: array([2], dtype=int32)

In [5]: df2 = pd.DataFrame(df)                                                                                                                                                  

In [6]: res2 = df2.notna()                                                                                                                                                      

In [7]: res2                                                                                                                                                                    
Out[7]: 
       A      B
0  False   True
1  False  False
2   True  False
3   True   True
4  False  False

In [8]: res2['A']                                                                                                                                                               
Out[8]: 
0    False
1    False
2     True
3     True
4    False
Name: A, dtype: bool
```

The result of `isna` / `notna` can be of type `Sparse[bool, False]` here as well.