Use nullable reference types

[bartdesmet]

No description provided.

[codecov]

Codecov Report

Merging #908 into master will increase coverage by 0.05%.
The diff coverage is 68.44%.

@@            Coverage Diff             @@
##           master     #908      +/-   ##
==========================================
+ Coverage   65.51%   65.57%   +0.05%     
==========================================
  Files         165      165              
  Lines       25429    25432       +3     
  Branches    18351    18311      -40     
==========================================
+ Hits        16661    16677      +16     
- Misses       2132     2135       +3     
+ Partials     6636     6620      -16

Flag	Coverage Δ
#ixnet	65.57% <68.44%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
...e/System.Linq.Async/System/Linq/AsyncEnumerator.cs	43.58% <ø> (ø)	⬆️
...roviders/System/Linq/AsyncQueryableEx.Generated.cs	99.79% <ø> (ø)	⬆️
...m.Linq.Async/System/Linq/AsyncEnumerableHelpers.cs	62.16% <ø> (ø)	⬆️
...c.Queryable/System/Linq/AsyncEnumerableRewriter.cs	55.9% <ø> (ø)	⬆️
...c.Queryable/System/Linq/AsyncEnumerableExecutor.cs	100% <ø> (ø)	⬆️
....Queryable/System/Linq/AsyncQueryable.Generated.cs	99.69% <ø> (-0.04%)	⬇️
...ource/System.Linq.Async/System/Linq/Disposables.cs	95.45% <ø> (ø)	⬆️
...Linq.Async/System/Linq/AsyncEnumerablePartition.cs	60.71% <0%> (ø)	⬆️
...m.Interactive.Async/System/Linq/Operators/MinBy.cs	36.92% <0%> (+1.02%)	⬆️
....Interactive.Async/System/Linq/Operators/Return.cs	59.61% <0%> (-3.85%)	⬇️
... and 77 more

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