## Summary
Allowed the config to not have nodes, links and agents and still be parsed
## Test process
Tested with config file that doesn't have any simulation key/value pair
## Checklist
- [X] PR is linked to a **work item**
- [X] **acceptance criteria** of linked ticket are met
- [X] performed **self-review** of the code
- [X] written **tests** for any new functionality added with this PR
- [ ] updated the **documentation** if this PR changes or adds functionality
- [ ] written/updated **design docs** if this PR implements new functionality
- [ ] updated the **change log**
- [X] ran **pre-commit** checks for code style
- [ ] attended to any **TO-DOs** left in the code
#2357 - Allowed the config to not have nodes, links and agents and still be parsed
Related work items: #2357