Return 200 empty payload for cached reads with empty result

src/Core/Resolvers/SqlQueryEngine.cs

@@ -443,6 +443,14 @@ public object ResolveList(JsonElement array, ObjectField fieldSchema, ref IMetad
 
         private static JsonDocument? ParseResultIntoJsonDocument(JsonElement? result)
         {
+            // An empty result set surfaces as a default (JsonValueKind.Undefined) JsonElement with no
+            // backing document; serializing it throws InvalidOperationException. Return null to match the
+            // non-cached path, which renders an empty payload (e.g. {"value":[]}).
+            if (result is { ValueKind: JsonValueKind.Undefined })
+            {
+                return null;
+            }
+
             byte[] jsonBytes = JsonSerializer.SerializeToUtf8Bytes(result);
             return JsonDocument.Parse(jsonBytes);
         }

src/Service.Tests/UnitTests/SqlQueryEngineHelperTests.cs

@@ -46,6 +46,25 @@ public void ParseResultIntoJsonDocument_HandlesValuesAndNull(string? json, bool
             Assert.AreEqual(hasObject ? JsonValueKind.Object : JsonValueKind.Null, result.RootElement.ValueKind);
         }
 
+        /// <summary>
+        /// Regression test for https://github.com/Azure/data-api-builder/issues/3704
+        /// An empty cached read surfaces as a default (JsonValueKind.Undefined) JsonElement that has no
+        /// backing document. Serializing it previously threw InvalidOperationException (HTTP 500).
+        /// ParseResultIntoJsonDocument must instead return null, matching the non-cached empty-result path.
+        /// </summary>
+        [TestMethod]
+        public void ParseResultIntoJsonDocument_UndefinedElement_ReturnsNull()
+        {
+            JsonElement? undefined = default(JsonElement);
+            MethodInfo method = typeof(SqlQueryEngine).GetMethod(
+                "ParseResultIntoJsonDocument",
+                BindingFlags.Static | BindingFlags.NonPublic)!;
+
+            JsonDocument? result = (JsonDocument?)method.Invoke(null, new object?[] { undefined });
+
+            Assert.IsNull(result);
+        }
+
         /// <summary>
         /// Verifies stored-procedure execution returns the first result object and maps empty or absent result arrays to null.
         /// </summary>