Fix test case for Shouldly 4

Author: pascalberger

src/Cake.AzureDevOps.Tests/Repos/PullRequest/CommentThread/AzureDevOpsPullRequestCommentThreadTests.cs

@@ -345,7 +345,6 @@ public void Should_Get_Valid_Properties()
                 prop1.ShouldNotBeEmpty();
                 prop1.ShouldBe("way");
 
-                prop2.ShouldNotBeNull();
                 prop2.ShouldBeOfType<int>();
                 prop2.ShouldBe(2);