Make sure GetTree() in mock reproduces behavior on GitLab (#825)

* Make sure GetTree() in mock reproduces behavior on GitLab

* Update test  in RepositoryMockTests.cs to match behavior

* Add comment in code to explain behavior

* Update NGitLab.Mock/Repository.cs

Co-authored-by: Louis Zanella <louis.zanella@gmail.com>

* Changes related to comment

---------

Co-authored-by: Louis Zanella <louis.zanella@gmail.com>

NGitLab.Mock.Tests/UsingVerify/RepositoryMockTests.cs

@@ -1,12 +1,10 @@
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
-using NGitLab.Mock.Clients;
 using NGitLab.Mock.Config;
 using NUnit.Framework;
 using static VerifyNUnit.Verifier;
 using RepositoryGetTreeOptions = NGitLab.Models.RepositoryGetTreeOptions;
-using Throws = NUnit.Framework.Throws;
 
 namespace NGitLab.Mock.Tests.UsingVerify;
 
@@ -26,10 +24,10 @@ public void Test_get_tree_when_subdirectory_does_not_exist_throws_not_found()
         var repository = client.GetRepository(1);
 
         // Act
-        var handler = () => repository.GetTreeAsync(new RepositoryGetTreeOptions { Path = "non-existing-directory" });
+        var treeCollection  = repository.GetTreeAsync(new RepositoryGetTreeOptions { Path = "non-existing-directory" });
 
         // Assert
-        Assert.That(handler, Throws.InstanceOf<GitLabNotFoundException>());
+        Assert.That(treeCollection , Is.Empty);
     }
 
     [Test]

NGitLab.Mock/Repository.cs

@@ -552,6 +552,11 @@ internal IEnumerable<Tree> GetTree(RepositoryGetTreeOptions repositoryGetTreeOpt
 
         if (tree is null)
         {
+            // As confirmed by the GetAllTreeObjectsAtInvalidPath() test, GitLab's behavior for
+            // GetTree() when Path doesn't exist is to return an empty collection.
+            if (!string.IsNullOrWhiteSpace(repositoryGetTreeOptions.Path))
+                return [];
+
             throw new GitLabNotFoundException();
         }