refactor(http-client-csharp): retain else-if for required nullable null fallback

Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>

Author: Copilot

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

@@ -2558,11 +2558,11 @@ private MethodBodyStatement CreateConditionalSerializationStatement(
                         _utf8JsonWriterSnippet.WriteNull(serializedName));
                 }
 
-                MethodBodyStatement writeNull = _utf8JsonWriterSnippet.WriteNull(serializedName);
-                return new IfElseStatement(
-                    condition,
-                    writePropertySerializationStatement,
-                    patchCheck != null ? new IfElseStatement(patchCheck, writeNull, null) : writeNull);
+                var ifStatement = new IfStatement(condition) { writePropertySerializationStatement };
+                var writeNull = _utf8JsonWriterSnippet.WriteNull(serializedName);
+                return patchCheck != null
+                    ? new IfElseStatement(ifStatement, [new IfStatement(patchCheck) { writeNull }], null)
+                    : new IfElseStatement(ifStatement, writeNull);
             }
 
             if (shouldCheckJsonPath)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/WriteRequiredNullableProperty.cs

@@ -41,12 +41,9 @@ protected virtual void JsonModelWriteCore(global::System.Text.Json.Utf8JsonWrite
                 writer.WritePropertyName("foo"u8);
                 writer.WriteStringValue(Foo);
             }
-            else
+            else if (!Patch.Contains("$.foo"u8))
             {
-                if (!Patch.Contains("$.foo"u8))
-                {
-                    writer.WriteNull("foo"u8);
-                }
+                writer.WriteNull("foo"u8);
             }
 
             Patch.WriteTo(writer);

packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/DynamicModel.Serialization.cs

@@ -221,12 +221,9 @@ protected virtual void JsonModelWriteCore(Utf8JsonWriter writer, ModelReaderWrit
                 Patch.WriteTo(writer, "$.requiredNullableDictionary"u8);
                 writer.WriteEndObject();
             }
-            else
+            else if (!Patch.Contains("$.requiredNullableDictionary"u8))
             {
-                if (!Patch.Contains("$.requiredNullableDictionary"u8))
-                {
-                    writer.WriteNull("requiredNullableDictionary"u8);
-                }
+                writer.WriteNull("requiredNullableDictionary"u8);
             }
             if (!Patch.Contains("$.primitiveDictionary"u8))
             {