fix: wrap missing DataLoader results in Result<T>.Resolve(default)

src/GreenDonut/src/GreenDonut/BatchDataLoader.cs

@@ -57,7 +57,7 @@ private static void CopyResults(
             }
             else
             {
-                results[i] = null;
+                results[i] = Result<TValue?>.Resolve(default);
             }
         }
     }
@@ -133,7 +133,7 @@ private static void CopyResults(
             }
             else
             {
-                results[i] = null;
+                results[i] = Result<TValue?>.Resolve(default);
             }
         }
     }

src/GreenDonut/test/GreenDonut.Tests/BatchDataLoaderTests.cs

@@ -69,6 +69,44 @@
         Assert.Equal(1, dataLoader.ExecutionCount);
     }
 
+    [Fact]
+    public async Task BatchDataLoader_MissingKey_Should_Be_ResultResolveDefault()
+    {
+        // arrange
+        var loader = new TestBatchLoader();
+        var keys = new[] { 1, 2 };
+        var results = new Result<string?>[2];
+
+        // act
+        await loader.FetchAsync(keys, results, default, CancellationToken.None);
+
+        Assert.Equal("one", results[0].Value);
+        Assert.Equal(ResultKind.Value, results[0].Kind);
+
+        // assert
+        Assert.Null(results[1].Value);
+        Assert.Equal(ResultKind.Value, results[1].Kind);
+    }
+
+    [Fact]
+    public async Task StatefulBatchDataLoader_MissingKey_Should_Be_ResultResolveDefault()
+    {
+        // arrange
+        var loader = new TestStatefulBatchLoader();
+        var keys = new[] { 1, 2 };
+        var results = new Result<string?>[2];
+
+        // act
+        await loader.FetchAsync(keys, results, default, CancellationToken.None);
+
+        Assert.Equal("one", results[0].Value);
+        Assert.Equal(ResultKind.Value, results[0].Kind);
+
+        // assert
+        Assert.Null(results[1].Value);
+        Assert.Equal(ResultKind.Value, results[1].Kind);
+    }
+
     [Fact]
     public async Task Null_Result()
     {
@@ -114,6 +152,35 @@
         }
     }
 
+    public class TestBatchLoader : BatchDataLoader<int, string>
+    {
+        public TestBatchLoader()
+            : base(new TestBatchScheduler(), new DataLoaderOptions()) { }
+
+        protected override Task<IReadOnlyDictionary<int, string>> LoadBatchAsync(
+            IReadOnlyList<int> keys,
+            CancellationToken cancellationToken)
+        {
+            return Task.FromResult<IReadOnlyDictionary<int, string>>(
+                new Dictionary<int, string> { { 1, "one" } });
+        }
+    }
+
+    public class TestStatefulBatchLoader : StatefulBatchDataLoader<int, string>
+    {
+        public TestStatefulBatchLoader()
+            : base(new TestBatchScheduler(), new DataLoaderOptions()) { }
+
+        protected override Task<IReadOnlyDictionary<int, string>> LoadBatchAsync(
+            IReadOnlyList<int> keys,
+            DataLoaderFetchContext<string> context,
+            CancellationToken cancellationToken)
+        {
+            return Task.FromResult<IReadOnlyDictionary<int, string>>(
+                new Dictionary<int, string> { { 1, "one" } });
+        }
+    }
+
     public sealed class InstantDispatcher : IBatchScheduler
     {
         public void Schedule(Batch batch)