Fix list add endpoint erroring

[cdrini]

List seeds can now optionally contain the thing field. This errors was the result of needing to use seed.get('key'), which errors in the JS. I apparently tried to fix this in my previous PR that introduced this issue but messed up the construct!

Technical

Testing

• ✅ Clicking "Create a list" button on my books page no longer errors
  • Note: There is an unrelated error from another pr on testing.openlibrary.org on the main books page, so you'll have to go to one of your sub pages, eg https://testing.openlibrary.org/account/lists
• ✅ Editing existing list works
• ✅ Adding notes works

Screenshot

Stakeholders

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 16.01%. Comparing base (21b78ad) to head (e267bbf).

Files	Patch %	Lines
openlibrary/plugins/openlibrary/js/jsdef.js	0.00%	1 Missing and 1 partial ⚠️
openlibrary/plugins/openlibrary/js/index.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8869      +/-   ##
==========================================
- Coverage   16.02%   16.01%   -0.02%     
==========================================
  Files          89       89              
  Lines        4693     4696       +3     
  Branches      818      820       +2     
==========================================
  Hits          752      752              
- Misses       3434     3436       +2     
- Partials      507      508       +1     

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[cdrini]

Scott gave a QA on his local environment and a preliminary review before his meeting, but I want to get this out with the deploy, so am auto merging. Scott will review after the fact!

[scottbarnes]

Looks good to me.