BUG: IntervalIndex([np.nan, np.nan]).is_monotonic returns True
I'm pretty sure this is related to other questionable behavior:

```
index = pd.IntervalIndex([np.nan, np.nan])
other = pd.IntervalIndex([np.nan])

In [4]: index.is_monotonic  # should be False
Out[4]: True

In [5]: index._index_as_unique  # should be False
Out[5]: True

In [6]: index.get_indexer_for(other)  # shouldn't raise
InvalidIndexError: Reindexing only valid with uniquely valued Index objects
```