Allow for empty properties in dotnet msbuild

[Zulu-Inuoe]

Fixes #4158

[Zulu-Inuoe]

@microsoft-github-policy-service agree

[augustoproiete]

Thanks for the PR @Zulu-Inuoe

Overall looks good, but there's at least one test failing (which expects the old behavior). Could you take a look and amend the test(s) (or delete if doesn't make sense anymore)?

Actual:   (null)
  Stack Trace:
     at Cake.Common.Tests.Unit.Tools.DotNet.MSBuild.DotNetMSBuildBuilderTests.TheBuildMethod.Should_Throw_If_Property_Has_No_Value(String[] propertyValues) in D:\a\1\s\src\Cake.Common.Tests\Unit\Tools\DotNet\MSBuild\DotNetMSBuildBuilderTests.cs:line 204
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
  Failed Cake.Common.Tests.Unit.Tools.DotNet.MSBuild.DotNetMSBuildBuilderTests+TheBuildMethod.Should_Throw_If_Property_Has_No_Value(propertyValues: [""]) [< 1 ms]
  Error Message:
   Assert.IsType() Failure
Expected: System.ArgumentException

[augustoproiete]

Hey @Zulu-Inuoe just following up on this one. Will you be able to get this to the finish line?