Missing data in proto maps

I have a theoretical proto file message defined as:
```
message Measurement {
double pressure = 1;
double temperature = 2;
}

message MeasurementMap {
map measurements = 1;
}

service WeatherStation {
rpc GetClimate (Empty) returns (MeasurementMap) {}
}
```

The problem is that if `MeasurementMap` contains a `Measurement` with all fields equal to 0, betterproto server replies with something like this:
```
{
"measurements": [
{
"key": "rpi1",
"value": {
"pressure": 1.01,
"temperature": 25.2,
}
},
{
"key": "rpi2"
}
]
}
```
Second key does not have any value next to it which makes _Postman_ throw a tantrum about "null value not having **pressure** field"

For comparison, here is the default behaviour of _grpclib_, which _Postman_ receives and parses A-OK:
```
{
"measurement": [
{
"key": "rpi1",
"value": {
"pressure": 1.01,
"temperature": 25.2
}
},
{
"key": "rpi2",
"value": {}
}
]
}
```

Comienza con la definición de proto del issue y reproduce una respuesta de WeatherStation que contenga una Measurement con valor cero en MeasurementMap. Compara la salida de betterproto para la entrada de mapa rpi2 con la salida de grpclib y, después, verifica que la respuesta incluya un objeto de valor vacío en lugar de omitirlo y que Postman lo analice correctamente.

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