chore: enable nullables for entire solution

[philasmar]

Issue #, if available:
DOTNET-8095

Description of changes:
Enable nullables for the entire solution

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[GarrettBeatty]

why these extra imports?

[philasmar]

After updating, ambiguous references started to show up. That's why these are needed.

[GarrettBeatty]

is null! a valid thing to do?

[philasmar]

This is used when we want to force a value to be null. It's fine to be used in certain cases. Also in the test suite it makes sense to avoid refactoring a lot and the tests would fail if anything unexpected happened.

[codecov]

Codecov Report

Attention: Patch coverage is 7.54717% with 49 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/net8-upgrade@6287d74). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...igFileDeployment/DeploymentSettingsHandlerTests.cs	0.00%	11 Missing ⚠️
...es/Validation/OptionSettingsItemValidationTests.cs	0.00%	11 Missing ⚠️
...tTests/LocalUserSettings/LocalUserSettingsTests.cs	0.00%	5 Missing ⚠️
...tTests/Recipes/Validation/ValidatorFactoryTests.cs	0.00%	5 Missing ⚠️
...tion/ECSFargateOptionSettingItemValidationTests.cs	0.00%	3 Missing ⚠️
...loymentManifestFile/DeploymentManifestFileTests.cs	0.00%	2 Missing ⚠️
...ation/AppRunnerOptionSettingItemValidationTests.cs	0.00%	2 Missing ⚠️
...ation/BlazorWasmOptionSettingItemValidationTest.cs	0.00%	2 Missing ⚠️
...ecipes/Validation/DockerfilePathValidationTests.cs	0.00%	2 Missing ⚠️
...lasticBeanStalkOptionSettingItemValidationTests.cs	0.00%	2 Missing ⚠️
... and 2 more

Additional details and impacted files

@@                   Coverage Diff                   @@
##             feature/net8-upgrade     #924   +/-   ##
=======================================================
  Coverage                        ?   62.34%           
=======================================================
  Files                           ?      294           
  Lines                           ?    10735           
  Branches                        ?     1562           
=======================================================
  Hits                            ?     6693           
  Misses                          ?     3483           
  Partials                        ?      559           

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