Apply suggestions from code review

Thanks @vbreuss applied your updates !

Co-authored-by: Valentin Breuß <vbreuss@gmail.com>

Author: Mpdreamz

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileSystemOptions.cs

@@ -16,8 +16,8 @@ public class MockFileSystemOptions
     public bool CreateDefaultTempDir { get; init; } = true;
 
     /// <summary>
-    ///     The temp directory returned by <see cref="System.IO.Path.GetTempPath()" /> on the <see cref="MockFileSystem" />.
+    ///     The temporary directory used by the <see cref="MockFileSystem" />.
     ///     Defaults to <see cref="System.IO.Path.GetTempPath()" /> when <see langword="null" /> or empty.
     /// </summary>
-    public string TemporaryDirectory { get; init; }
+    public string? TemporaryDirectory { get; init; }
 }