Handle parsing fields containing an empty relation list in and out of storage

[sandreae]

#334

In order to do this we make value on operation and document field rows optional and interpret this to mean an empty relation list when parsing values from the store.

Then this threw up another bug which i believe originates in p2panda-rs where we don't handle decoding pinned relation lists if they contain no items. p2panda/p2panda#489

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• Link this PR to any issues it closes
• New files contain a SPDX license header

[codecov]

Codecov Report

Patch coverage: 98.85% and project coverage change: +0.26 🎉

Comparison is base (3da21cb) 89.27% compared to head (b3d4cc0) 89.53%.

❗ Current head b3d4cc0 differs from pull request most recent head 88bc6fc. Consider uploading reports for the commit 88bc6fc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #336      +/-   ##
==========================================
+ Coverage   89.27%   89.53%   +0.26%     
==========================================
  Files          70       68       -2     
  Lines        5881     5143     -738     
==========================================
- Hits         5250     4605     -645     
+ Misses        631      538      -93     

Impacted Files	Coverage Δ
aquadoggo/src/graphql/queries/next_args.rs	97.43% <ø> (ø)
aquadoggo/src/db/models/utils.rs	99.42% <98.54%> (-0.37%)	⬇️
aquadoggo/src/graphql/mutations/publish.rs	100.00% <100.00%> (ø)
aquadoggo/src/materializer/tasks/dependency.rs	98.96% <100.00%> (+<0.01%)	⬆️
aquadoggo/src/test_utils/helpers.rs	97.05% <100.00%> (+0.08%)	⬆️

... and 25 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.