Hotfix/optional leaf object

[oytuntez]

This PR aims to improve DocList fields that are Optional or that are given None default value. Giving empty [] as default value would create entries in the subindex (with just the ID field, no other data), so I opted for None.

The biggest issue that this PR aims to solve is DocList type fields would be required no matter if they have a default value or are marked as Optional.

[JoanFM]

Thanks for the PR, is there a chance for you to add tests for the fixes? Also, I see code related to ES, does it belong to the same problem?

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7c1e18e) 84.68% compared to head (0a7e030) 84.67%.
Report is 2 commits behind head on main.

❗ Current head 0a7e030 differs from pull request most recent head 7555eb3. Consider uploading reports for the commit 7555eb3 to get more accurate results

Files	Patch %	Lines
docarray/index/backends/elastic.py	66.66%	1 Missing ⚠️
docarray/index/backends/hnswlib.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1865      +/-   ##
==========================================
- Coverage   84.68%   84.67%   -0.02%     
==========================================
  Files         136      136              
  Lines        9261     9268       +7     
==========================================
+ Hits         7843     7848       +5     
- Misses       1418     1420       +2     

Flag	Coverage Δ
docarray	84.67% <87.50%> (-0.02%)	⬇️

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[JohannesMessner]

+1 for adding a test, once there is a test I could take over for the review if you want @JoanFM

[JoanFM]

+1 for adding a test, once there is a test I could take over for the review if you want @JoanFM

Perfect!

[JoanFM]

Also @oytuntez, please be aware that we will need to have all the commits signed-off for us to merge the contributions.