refactor: enable nullable ref type checking

[natemcmaster]

Prior to this, some nullability annotations were incorrect.

⚠️ This is technically a breaking change on a few public methods where ValidationResult! has been changed to ValidationResult?. The previous nullability annotation were incorrect. See the PublicAPI.Shipped.txt changes for a full list of impacted APIs.

[codecov]

Codecov Report

Merging #425 (d9d9994) into main (db2b121) will decrease coverage by 0.12%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
- Coverage   67.84%   67.72%   -0.13%     
==========================================
  Files         105      106       +1     
  Lines        3281     3284       +3     
==========================================
- Hits         2226     2224       -2     
- Misses       1055     1060       +5     

Impacted Files	Coverage Δ
...mandLineUtils/CommandLineApplication.Validation.cs	95.74% <ø> (ø)
src/CommandLineUtils/CommandLineApplication{T}.cs	90.90% <ø> (ø)
...mandLineUtils/HelpText/DefaultHelpTextGenerator.cs	94.73% <ø> (ø)
...l/ValueParsers/TypeDescriptorValueParserFactory.cs	84.21% <ø> (ø)
.../CommandLineUtils/Properties/NullabilityHelpers.cs	0.00% <0.00%> (ø)
.../CommandLineUtils/Validation/AttributeValidator.cs	82.60% <ø> (ø)
...Utils/Attributes/FilePathNotExistsAttributeBase.cs	71.42% <50.00%> (ø)
...mandLineUtils/Attributes/AllowedValuesAttribute.cs	66.66% <100.00%> (ø)
...ineUtils/Attributes/FilePathExistsAttributeBase.cs	73.68% <100.00%> (ø)
...mandLineUtils/Attributes/LegalFilePathAttribute.cs	100.00% <100.00%> (ø)
... and 12 more

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