fix(jsonpopulator) Updated processMapType to handle nested abstract c…

[kshitij79]

…lasses - I769

Closes #769

Changes

• This is a minor pull request and lacks test cases here https://github.com/accordproject/concerto/blob/main/packages/concerto-core/test/serializer/jsonpopulator.js
• This is my first PR (Hence review is needed)

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

[dselman]

Please add a unit test. Code looks good.

[mttrbrts]

Great contribution! Thanks @kshitij79 🎉

[DianaLease]

This may have been fixed in v4.x as the keys and values of maps are now treated as fields (change by @dselman ), but we should still merge this to get this fix and it's tests in now 👍

[github-actions]

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