use field default-factory for optional dict fields

[leifdenby]

Describe your changes

This PR fixes a bug with the config specification for fields that are of dict type but are optional. I had previously thought it was ok to set these to None by default, but that doesn't work because when constructing the nested dataclasses the initialisation tries to iterate over the .items() in these optional fields. Since these fields were defaulting to None this would raise an AttributeError. Instead for fields of dict type where these fields are optional one must use a dataclass.field(default_factory={}) constructor so that an empty dict is created when intialising the dataclass object.

I think @mafdmi noticed this earlier and has already fixed some of these issues, but I spotted a few more just now when working on the config serialisation feature #46.

No dependencies change

Issue Link

No related issue

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• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
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[mafdmi]

Great to fix this:)

[leifdenby]

ok. I think I have fixed the things you commented on @mafdmi :) thank you for the review! I'm really glad you looked at this because I clearly missed some things. Hopefully you think this looks ok now :)

[mafdmi]

Looks great!