PresetDefinition.with_additional_config bugfix

[esztermarton]

Replacing self.run_config with {} pre merge_dicts if self.run_config is falsey

Summary

Bugfix.
Related ticket has all info on context: #3985

I found minimal change to be to add
initial_config = self.run_config or {}
This will also ensure that the new config is a new dict (rather than the same dict object that was passed in).

Happy for alternative suggestions.

Test Plan

Added happy path test to assert that even if initial run_config is None, the with_additional_config won't fail and that a new object will be returned and that it will have expected config.

Have considered also adding test case permutations where the config-to-be-added is a dict vs None - however there would be a problem with the assertion that the object is always a new object (which atm would not be the case if the new_runconfig is None). This test would align with docstring - but it would require more code changes and would also change current behaviour.

Checklist

• My change requires a change to the documentation and I have updated the documentation accordingly.
• I have added tests to cover my changes.

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[alangenfeld]

thanks!