Fix `nullable` warnings when building test code

[dougbu]

UsePathBaseExtensionsTests.cs(57,62): warning CS8613: Nullability of reference types in return type of 'IDictionary<string, object> ApplicationBuilderWrapper.Properties.get' doesn't match implicitly implemented member 'IDictionary<string, object?> IApplicationBuilder.Properties.get'. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
UseWhenExtensionsTests.cs(114,60): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(51,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(52,72): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(66,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(78,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(90,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPredicateMiddlewareTests.cs(102,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(42,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(43,72): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(60,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(88,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(116,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(132,92): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(146,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(168,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
MapPathMiddlewareTests.cs(182,67): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
UsePathBaseExtensionsTests.cs(166,60): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
UsePathBaseExtensionsTests.cs(57,62): warning CS8619: Nullability of reference types in value of type 'IDictionary<string, object?>' doesn't match target type 'IDictionary<string, object>'. [...\src\Http\Http.Abstractions\test\Microsoft.AspNetCore.Http.Abstractions.Tests.csproj]
DefaultAntiforgeryTokenStoreTest.cs(167,40): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Antiforgery\test\Microsoft.AspNetCore.Antiforgery.Test.csproj]
DefaultAntiforgeryTokenStoreTest.cs(194,40): warning CS8625: Cannot convert null literal to non-nullable reference type. [...\src\Antiforgery\test\Microsoft.AspNetCore.Antiforgery.Test.csproj]

[dougbu]

Part of #22240

[javiercn]

/cc: @pranavkm

[dougbu]

/ping @pranavkm should this be assigned to you❔

[dougbu]

I gave this a bad title because we don't compile test code on the CI and the warnings occur only in test code. Fixing that…