fix(core): return envelope if no sinks in dispatcher

[jrriehl]

Proposed Changes

Restores the functionality of getting a return envelope when sending a sync message from a standalone function, as in https://github.com/fetchai/uAgents/blob/main/python/examples/08-local-network-interaction/sync_sender.py.

Types of changes

What type of change does this pull request make (put an x in the boxes that apply)?

• [x] Bug fix (non-breaking change that fixes an issue).
• [ ] New feature added (non-breaking change that adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• [ ] Documentation update.
• [ ] Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

Put an x in the boxes that apply:

• [x] I have read the CONTRIBUTING guide
• [x] Checks and tests pass locally

If applicable

• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked that code coverage does not decrease
• [ ] I have added/updated the documentation

Further comments

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