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.

Beginne damit, die für Message.to_dict verwendete timedelta-Behandlung zu lokalisieren und die beiden Beispiele aus dem Issue unter Python 3.11 zu reproduzieren. Überprüfe, dass eine explizit angegebene timedelta von null Sekunden im Dictionary erhalten bleibt und die Ausgabe dem erwarteten Fall {'timeout': '0s'} entspricht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.