Query params MultiDict.getlist raises KeyError on missing key

```python
@app.route('/')
def index():
query_params = app.current_request.query_params
stuff = query_params.getlist("stuff") # KeyError if query param not passed in request
```

I believe that it makes sense that a behavior similar to using `get` (fall back to a value) is expected when using `getlist`.

Start by locating the MultiDict implementation and comparing getlist with get, then inspect existing tests for missing query parameters. Clarify the expected fallback value for a missing key, implement the agreed behavior, and add or update a regression test showing that getlist no longer raises KeyError.

Written by the indexing model from the issue text.