chore: code review changes

Author: christianoMicrosoft

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -437,7 +437,9 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             });
 
             // enum
-            var enumValue = !string.IsNullOrEmpty(Const) && version < OpenApiSpecVersion.OpenApi3_1
+            var enumValue = (Enum == null || Enum.Count == 0)
+                && !string.IsNullOrEmpty(Const) 
+                && version < OpenApiSpecVersion.OpenApi3_1
                 ? new List<JsonNode> { JsonValue.Create(Const)! }
                 : Enum;
             writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
@@ -690,7 +692,7 @@ private void SerializeAsV2(
             });
 
             // enum
-            var enumValue = !string.IsNullOrEmpty(Const)
+            var enumValue = (Enum == null || Enum.Count == 0) && !string.IsNullOrEmpty(Const)
                 ? new List<JsonNode> { JsonValue.Create(Const)! }
                 : Enum;
             writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -659,7 +659,8 @@ public async Task SerializeConstAsEnumV30()
 
             // Assert
             var v3Node = JsonNode.Parse(actual);
-            Assert.True(v3Node!["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.NotNull(v3Node);
+            Assert.True(v3Node["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
             Assert.False(v3Node.AsObject().ContainsKey("const"));
         }
 
@@ -678,7 +679,8 @@ public async Task SerializeConstAsEnumV20()
 
             // Assert
             var v2Node = JsonNode.Parse(actual);
-            Assert.True(v2Node!["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.NotNull(v2Node);
+            Assert.True(v2Node["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
             Assert.False(v2Node.AsObject().ContainsKey("const"));
         }