BUG: MultiIndex.putmask

```
left = pd.MultiIndex.from_tuples([(np.nan, 6), (np.nan, 6), ("a", 4)])
right = pd.MultiIndex.from_tuples([("a", 1), ("a", 1), ("d", 1)])
mask = np.array([True, True, False])

result = left.putmask(mask, right)
>>> result
[...]
IndexError: list index out of range
```

The result here somehow ended up with `len(result.levels) == 0`