Accommodate nil values correctly in JSON param fetching

[stephendolan]

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Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[jwoertink]

This is probably fine. I think to really test it, we would have to tell Avram to point to main branch, then write the spec in Avram that tests this whole deal.

[stephendolan]

@jwoertink I actually pulled this functionality down into the helper so there's no inconsistency when this is used by other methods in the same file. It still fixes the API client failure I was seeing and passes our test suite. 🥳

Agreed that we should test this in some way, but I'm not even sure where to start :(

[jwoertink]

Good enough for me! Thanks for fixing it 🚀