Handle KeyError: 'filePicker' while editing assignments

Sentry Issue: [LMS-BACKEND-SY](https://hypothesis.sentry.io/issues/4145279828/?referrer=github_integration)

```
KeyError: 'filePicker'
(11 additional frame(s) were not displayed)
...
File "lms/validation/__init__.py", line 92, in wrapper_view
return view(context, request)
File "pyramid/viewderivers.py", line 427, in rendered_view
result = view(context, request)
File "newrelic/hooks/framework_pyramid.py", line 157, in _wrapper
return wrapper(context, request)
File "pyramid/viewderivers.py", line 129, in _class_view
response = getattr(inst, attr)()
File "lms/views/lti/basic_launch.py", line 109, in reconfigure_assignment_config
"filePicker": config["filePicker"],
```