feat/improved json

- Automatic data update - Produce empty lists and new aa_nuc_mappings ### Description of proposed changes What is the goal of this pull request? What does this pull request change? ### Related issue(s) <!-- Start typing the name of a related issue and github will autosuggest the issue number for you --> Fixes # Related to # ### Testing What steps should be taken to test the changes you've proposed? If you added or changed behavior in the codebase, did you update the tests, or do you need help with this? ### Thank you for contributing to Nextstrain!