Fix Utf8JsonWriterHelper to not ignore null dictionary values

[david-driscoll]

There is a problem with the Utf8JsonWriterHelper such that if a null value is given to a dictionary it never writes the key and the null value in the resulting json.

For example, this would avoid sending any null values from StrawberryShake in such a way that if you wanted to set a null value on a request, you could not. They will always come back as unassigned on the server side and it would never see the null value given.

Closes #6681

NOTE: this and #6683 both exist in (at least) 13.7.0 as well, and should likely be hot fixed.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (6965960) 78.98% compared to head (13aaf5e) 76.55%.
Report is 72 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6689      +/-   ##
==========================================
- Coverage   78.98%   76.55%   -2.44%     
==========================================
  Files        2903     2541     -362     
  Lines      139771   127135   -12636     
==========================================
- Hits       110397    97326   -13071     
- Misses      29374    29809     +435     

Flag	Coverage Δ
unittests	76.55% <85.18%> (-2.44%)	⬇️

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

Files	Coverage Δ
...e/src/CookieCrumble/Extensions/WriterExtensions.cs	100.00% <ø> (ø)
...CookieCrumble/src/CookieCrumble/TestEnvironment.cs	0.00% <ø> (ø)
...polloFederationRequestExecutorBuilderExtensions.cs	66.66% <100.00%> (ø)
...ensions/ApolloFederationSchemaBuilderExtensions.cs	86.66% <ø> (ø)
.../src/ApolloFederation/FederationTypeInterceptor.cs	100.00% <100.00%> (ø)
...ion/src/ApolloFederation/Helpers/ArgumentParser.cs	68.65% <100.00%> (+0.47%)	⬆️
...rc/AspNetCore.CommandLine/Command/ExportCommand.cs	100.00% <100.00%> (ø)
...tChocolateAspNetCoreServiceCollectionExtensions.cs	93.33% <ø> (+26.66%)	⬆️
...rc/AspNetCore/Extensions/HttpResponseExtensions.cs	26.31% <ø> (+4.57%)	⬆️
...pNetCore/src/AspNetCore/GraphQLRequestException.cs	25.00% <ø> (+5.00%)	⬆️
... and 14 more

... and 797 files with indirect coverage changes

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[michaelstaib]

Thank you again for all your help.