Fix nullExamples

[jgarciadelanoceda]

Fixes #3801.
As previously the null was being always serialized and also keep compatibility with not nullable references

As the integer/double could be always have the nullableReference because the objects where int?,double? for old versions seems fine.
I also tought about the object case, that for projects that do not have the nullable references enabled seems OK to me to have that case and also for the case of strings.. I do not really know if I should also place an specific case for arrays

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (8ad6682) to head (357931c).
⚠️ Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3803      +/-   ##
==========================================
- Coverage   94.93%   94.92%   -0.01%     
==========================================
  Files         111      111              
  Lines        3886     3885       -1     
  Branches      784      783       -1     
==========================================
- Hits         3689     3688       -1     
  Misses        197      197              

Flag	Coverage Δ
Linux	94.92% <100.00%> (-0.01%)	⬇️
Windows	94.92% <100.00%> (-0.01%)	⬇️
macOS	94.92% <100.00%> (-0.01%)	⬇️

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[jgarciadelanoceda]

@martincostello could you review this?, I think that there is an extra property that is the format that is right now no neccesary but I did at the first implementation needed(I could remove it by the wat)

[martincostello]

could you review this?

I see all issues, PRs and comments in this repo - I get to stuff when it fits into my day and my TODO/priority list 😃

[martincostello]

FYI I'm going to be away next week, so even if this PR is approved/merged today, any new version won't ship until after I'm back as this is already the third attempt at resolving the underlying issue and I don't want a fourth attempt looming while I'm away 😄

[martincostello]

Could we add an integration test for this too?

[martincostello]

Once there's some user feedback that this looks right (just to avoid too many releases trying to fix this bug) then can look at merging.

Seems reasonable to me.