fix: Treat explicitly set nil values like omitted values

[AndrewSisley]

Relevant issue(s)

Resolves #3100

Description

Treat explicitly set nil values like omitted values.

There were a couple of bugs here, I suggest reviewing commit by commit as the old behaviour was documented before being corrected.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.73%. Comparing base (4e5470c) to head (e209fa6).

Files with missing lines	Patch %	Lines
client/document.go	64.29%	3 Missing and 2 partials ⚠️
internal/core/crdt/lwwreg.go	50.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3101      +/-   ##
===========================================
- Coverage    79.81%   79.73%   -0.08%     
===========================================
  Files          351      351              
  Lines        27335    27351      +16     
===========================================
- Hits         21816    21808       -8     
- Misses        3982     3998      +16     
- Partials      1537     1545       +8     

Flag	Coverage Δ
all-tests	79.73% <62.50%> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
internal/db/collection.go	74.12% <100.00%> (+0.50%)	⬆️
internal/core/crdt/lwwreg.go	76.27% <50.00%> (-1.51%)	⬇️
client/document.go	70.12% <64.29%> (-0.27%)	⬇️

... and 11 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4e5470c...e209fa6. Read the comment docs.

[nasdf]

LGTM