Unexpected behaviour with explicitly setting seconds = 0 with timedelta and using to_dict

### Summary

When using timedelta with explicitly setting the seconds to 0 the output is empty instead of being explicitly set

### Reproduction Steps

```python3
>>> from datetime import timedelta
>>> from dataclasses import dataclass
>>> import betterproto
>>>
>>> @dataclass(eq=False, repr=False)
... class TestTimeout(betterproto.Message):
... timeout: timedelta = betterproto.message_field(1)
...
>>> TestTimeout(timeout=timedelta(seconds=0)).to_dict()
{}
>>> from datetime import timedelta
>>> from dataclasses import dataclass
>>> import betterproto
>>>
>>> @dataclass(eq=False, repr=False)
... class TestTimeout(betterproto.Message):
... timeout: timedelta = betterproto.message_field(1)
...
>>> TestTimeout(timeout=timedelta(seconds=1)).to_dict()
{'timeout': '1.000s'}
```

### Expected Results

Output expected to be `{'timeout': '0s'}`

### Actual Results

Output is an empty dict {}

### System Information

betterproto==2.0.0b6

python 3.11

### 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.

Comienza localizando el manejo de timedelta utilizado por Message.to_dict y reproduce los dos ejemplos del issue en Python 3.11. Comprueba que una timedelta de cero segundos proporcionada explícitamente se conserve en el diccionario, con una salida que coincida con el caso esperado {'timeout': '0s'}.

Escrito por el modelo de indexación a partir del texto del issue.