Main issues:
* `AIStructuredResponseGeneratorBlock` is not able to produce a list of
objects.
* `SmartDecisionBlock` is not able to call tools with some optional
inputs.
### Changes 🏗️
* Allow persisting `null` / `None` value as execution output.
* Provide `multiple_tool_calls` option for `SmartDecisionBlock`.
* Provide `list_result` option for `AIStructuredResponseGeneratorBlock`
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
<!-- Put your test plan here: -->
- [x] Run `SmartDecisionBlock` & `AIStructuredResponseGeneratorBlock`
3df6dcd26b