feat: Allow setting null values on doc fields

[islamaliev]

Relevant issue(s)

Resolves #2272

Description

Allows setting of null values for nillable fields.

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Integration tests.

Specify the platform(s) on which this was tested:

• MacOS

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (5834f81) 74.04% compared to head (3ce2d75) 74.07%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2273      +/-   ##
===========================================
+ Coverage    74.04%   74.07%   +0.03%     
===========================================
  Files          256      256              
  Lines        25748    25761      +13     
===========================================
+ Hits         19063    19080      +17     
+ Misses        5367     5362       -5     
- Partials      1318     1319       +1     

Flag	Coverage Δ
all-tests	74.07% <78.26%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
client/ctype.go	93.10% <100.00%> (ø)
core/encoding.go	64.23% <100.00%> (ø)
merkle/crdt/merklecrdt.go	87.80% <100.00%> (ø)
planner/sum.go	88.16% <100.00%> (ø)
request/graphql/schema/collection.go	90.95% <100.00%> (+0.50%)	⬆️
client/document.go	67.85% <83.33%> (+0.25%)	⬆️
db/index.go	63.24% <50.00%> (ø)
client/descriptions.go	65.91% <42.86%> (ø)

... and 9 files with indirect coverage changes

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[fredcarle]

LGTM! Thanks for putting this together.