OneOf and empty Message issue

When try to use empty Message (Message without fields) in one_of group, I cannot convert it to dict or bool.

Code to reproduce:
```proto
syntax = "proto3";

message CommandGetInfo
{}

message CommandGetName
{}

message Test {
oneof foo {
bool on = 1;
int32 count = 2;
string name = 3;
CommandGetInfo get_info = 4;
CommandGetName get_name = 5;
}
}
```

```python
import betterproto
from oneof import Test, CommandGetInfo, CommandGetName

def print_one_of(msg: Test):
def _to_bool(_value) -> bool:
return True if _value else False
msg_one_of = betterproto.which_one_of(msg, "foo")
print(f"msg={msg}")
print(f"msg_one_of={msg_one_of}")
print(f"msg to dict={msg.to_dict()}")
print(f"to bool test:")
print(f"\tmsg.on={_to_bool(msg.on)}")
print(f"\tmsg.count={_to_bool(msg.count)}")
print(f"\tmsg.name={_to_bool(msg.name)}")
print(f"\tmsg.get_info={_to_bool(msg.get_info)}")
print(f"\tmsg.get_name={_to_bool(msg.get_name)}")
print(f"to bytes: {bytes(msg)}")
print("\n")

test = Test()
print_one_of(msg=test)

test.get_name = CommandGetName()
print_one_of(msg=test)

test.on = True
print_one_of(msg=test)

test.count = 57
print_one_of(msg=test)

test.name = "Name"
print_one_of(msg=test)

test.get_info = CommandGetInfo()
print_one_of(msg=test)
```

Output:
```bash
msg=Test(on=False, count=0, name='', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('', None)
msg to dict={}
to bool test:
msg.on=False
msg.count=False
msg.name=False
msg.get_info=True
msg.get_name=True
to bytes: b''

msg=Test(on=False, count=0, name='', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('get_name', CommandGetName())
msg to dict={}
to bool test:
msg.on=False
msg.count=False
msg.name=False
msg.get_info=True
msg.get_name=True
to bytes: b''

msg=Test(on=True, count=0, name='', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('on', True)
msg to dict={'on': True}
to bool test:
msg.on=True
msg.count=False
msg.name=False
msg.get_info=True
msg.get_name=True
to bytes: b'\x08\x01'

msg=Test(on=False, count=57, name='', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('count', 57)
msg to dict={'count': 57}
to bool test:
msg.on=False
msg.count=True
msg.name=False
msg.get_info=True
msg.get_name=True
to bytes: b'\x109'

msg=Test(on=False, count=0, name='Name', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('name', 'Name')
msg to dict={'name': 'Name'}
to bool test:
msg.on=False
msg.count=False
msg.name=True
msg.get_info=True
msg.get_name=True
to bytes: b'\x1a\x04Name'

msg=Test(on=False, count=0, name='', get_info=CommandGetInfo(), get_name=CommandGetName())
msg_one_of=('get_info', CommandGetInfo())
msg to dict={}
to bool test:
msg.on=False
msg.count=False
msg.name=False
msg.get_info=True
msg.get_name=True
to bytes: b''
```

We can see that bool values are always set to True for empty proto messages, serializing them to dictionary gives empty dictionary, serializing to bytes also gives empty value.

I tested on python versions 3.9 and 3.10.
Betterproto: 1.2.5