fix(json): need to write null when structure is null

codegen/src/serialization/json.zig

@@ -213,6 +213,8 @@ fn writeStructureJson(params: WriteMemberJsonParams, writer: std.io.AnyWriter) !
             try writer.writeAll("try jw.endObject();\n");
 
             if (is_optional) {
+                try writer.writeAll("} else {\n");
+                try writer.writeAll("try jw.write(null);\n");
                 try writer.writeAll("}\n");
             }
         }

src/aws.zig

@@ -2703,6 +2703,8 @@ test "jsonStringify" {
 }
 
 test "jsonStringify nullable object" {
+    // structure is not null
+    {
         const request = services.lambda.CreateAliasRequest{
             .function_name = "foo",
             .function_version = "bar",
@@ -2730,3 +2732,24 @@ test "jsonStringify nullable object" {
         try testing.expectEqualStrings("baz", json_parsed.value.Name);
         try testing.expectEqual(null, json_parsed.value.RoutingConfig.AdditionalVersionWeights);
     }
+
+    // structure is null
+    {
+        const request = services.kms.DecryptRequest{
+            .key_id = "foo",
+            .ciphertext_blob = "bar",
+        };
+
+        const request_json = try std.json.stringifyAlloc(std.testing.allocator, request, .{});
+        defer std.testing.allocator.free(request_json);
+
+        const json_parsed = try std.json.parseFromSlice(struct {
+            KeyId: []const u8,
+            CiphertextBlob: []const u8,
+        }, testing.allocator, request_json, .{ .ignore_unknown_fields = true });
+        defer json_parsed.deinit();
+
+        try testing.expectEqualStrings("foo", json_parsed.value.KeyId);
+        try testing.expectEqualStrings("bar", json_parsed.value.CiphertextBlob);
+    }
+}