Map Adapter cannot handle Value whose representation can be zero bytes

Subj

If you use message like:

message ValidZeroPayload {
    float distance_meters = 2;
    reserved 1;
}

Wire will choke on parsing map with value initialized as:

ValidZeroPayload {
    distance_meters: 0.0
}

That's because it will be encoded as zero bytes due to zero being treated as non-presence in case of integers and floats

Therefore it triggers this check to fail:
https://github.com/square/wire/blob/master/wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt#L812
Even though message itself is legit in protobuf 3

Start in wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt at the check around line 812, and reproduce the issue with the ValidZeroPayload protobuf example. Verify that a map value whose valid protobuf representation is zero bytes can be parsed successfully, while malformed input remains rejected.