BUG: Series.get() does not return default value in case one of slice(n,m) is not found
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
ds = pd.Series(['a', 'b', 'c'])
print(ds.get([1,-1]))
print(ds.get(slice(-1,2)))

```

#### Problem description

The first get call returns the default value None as expected. The second returns an empty series. This seems inconsistent. I propose that we return here None too.

#### Expected Output

None and None

#### Output of ``pd.show_versions()``

<details>

master

</details>
