BUG: crosstab with margins=True and dropna=False
xref #12614 

```
In [1]: df = pd.DataFrame({'a': [1, 2, 2, 2, 2, np.nan],'b': [3, 3, 4, 4, 4, 4]})

In [2]: pd.crosstab(df.a, df.b, margins=True, dropna=True)
Out[2]: 
b    3  4  All
a             
1.0  1  0    1
2.0  1  3    4
All  2  3    5

In [3]: pd.crosstab(df.a, df.b, margins=True, dropna=False)
KeyError: 'Level None not found'
```
