Nested message with repeated field not in to_json when mutated after initialization

### Summary

Possibly related to #199

Given a message with a nested message field containing a repeated field, if the field is mutated (eg. via `list.append`) after initialization, `to_dict` will not include it.

### Reproduction Steps

Given a proto schema like this

```protobuf
syntax = "proto3";

message RootMessage {
NestedMessage nested = 1;
}

message NestedMessage {
repeated string content = 1;
}
```

Running the following script

```python
message = RootMessage()

message.nested.content.append("test-content")

print("Including default values: True")
pprint(message.to_dict(include_default_values=True))

print("Including default values: False")
pprint(message.to_dict(include_default_values=False))
```

will result in:

```
Including default values: True
{'nested': {'content': ['test-content']}}

Including default values: False
{}
```

### Expected Results

I would expect the `content` field of the `nested` message to appear in the returned `dict` also when `include_default_values` is set to False, as its content was mutated and it's no longer the default value.

### Actual Results

`to_dict` returns an empty dict

### System Information

```
libprotoc 3.21.6
Python 3.10.13
Name: betterproto
Version: 2.0.0b6
Summary: A better Protobuf / gRPC generator & library
Home-page: https://github.com/danielgtaylor/python-betterproto
Author: Daniel G. Taylor
Author-email: danielgtaylor@gmail.com
License: MIT
Location: /home/marco/.local/share/rtx/installs/python/3.10.13/lib/python3.10/site-packages
Requires: grpclib, python-dateutil
Required-by:
```

### Checklist

- [X] I have searched the issues for duplicates.
- [X] I have shown the entire traceback, if possible.
- [X] I have verified this issue occurs on the latest prelease of betterproto which can be installed using `pip install -U --pre betterproto`, if possible.