Fix build issues from new Moq version

tests/Jellyfin.Api.Tests/Auth/DefaultAuthorizationPolicy/DefaultAuthorizationHandlerTests.cs

@@ -66,7 +66,7 @@ namespace Jellyfin.Api.Tests.Auth.DefaultAuthorizationPolicy
 
             _userManagerMock
                 .Setup(u => u.GetUserById(It.IsAny<Guid>()))
-                .Returns<User?>(null);
+                .Returns<User?>(_ => null);
 
             var claims = new[]
             {

tests/Jellyfin.Server.Implementations.Tests/Library/LibraryManager/FindExtrasTests.cs

@@ -35,7 +35,7 @@ public class FindExtrasTests
         var configMock = fixture.Freeze<Mock<IServerConfigurationManager>>();
         configMock.Setup(c => c.ApplicationPaths.ProgramDataPath).Returns("/data");
         var itemRepository = fixture.Freeze<Mock<IItemRepository>>();
-        itemRepository.Setup(i => i.RetrieveItem(It.IsAny<Guid>())).Returns<BaseItem>(null);
+        itemRepository.Setup(i => i.RetrieveItem(It.IsAny<Guid>())).Returns<BaseItem>(null!);
         _fileSystemMock = fixture.Freeze<Mock<IFileSystem>>();
         _fileSystemMock.Setup(f => f.GetFileInfo(It.IsAny<string>())).Returns<string>(path => new FileSystemMetadata { FullName = path });
         _libraryManager = fixture.Build<Emby.Server.Implementations.Library.LibraryManager>().Do(s => s.AddParts(