Fix empty fields

[bart-degreed]

Allows incoming empty fieldset (json:api spec compliance). The code adds a default interface property to not break existing implementations. Includes a workaround for bug dotnet/aspnetcore#33394.

Fixes #996.

[codecov]

Codecov Report

Merging #1009 (afb02c0) into master (56c3416) will decrease coverage by 0.13%.
The diff coverage is 77.94%.

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
- Coverage   92.01%   91.87%   -0.14%     
==========================================
  Files         283      286       +3     
  Lines        7661     7718      +57     
==========================================
+ Hits         7049     7091      +42     
- Misses        612      627      +15     

Impacted Files	Coverage Δ
.../JsonApiDotNetCore/Middleware/FixedQueryFeature.cs	59.09% <59.09%> (ø)
.../JsonApiDotNetCore/Middleware/FixedQueryHelpers.cs	80.00% <80.00%> (ø)
.../JsonApiDotNetCore/Middleware/JsonApiMiddleware.cs	100.00% <100.00%> (ø)
...e/Queries/Internal/Parsing/SparseFieldSetParser.cs	100.00% <100.00%> (ø)
...etCore/QueryStrings/IQueryStringParameterReader.cs	100.00% <100.00%> (ø)
...NetCore/QueryStrings/Internal/QueryStringReader.cs	100.00% <100.00%> (ø)
...ternal/SparseFieldSetQueryStringParameterReader.cs	100.00% <100.00%> (ø)

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