Fix bug in proto2json map empty keys serialization

`<library/cpp/protobuf/json/proto2json.h>` неправильно сериализует пустой ключ protobuf'ной map'ы

```
message TMyMessage {
    map<string, string> Fields = 1;
}
```

При таком протобуфе

```
Fields { key: "" value: "1" }
```

Сейчас получается json

```
{
    "value": "1"
}
```

Хотя должен получаться такой

```
{
    "": 1
}
```
commit_hash:e678bffbda1a062fa0fcaaba65c2e5f6bf13d558

1 files changed, 1 insertions, 1 deletions

diff --git a/library/cpp/protobuf/json/proto2json_printer.cpp b/library/cpp/protobuf/json/proto2json_printer.cpp
index a9f8c3fce99..706b3b8c628 100644
--- a/library/cpp/protobuf/json/proto2json_printer.cpp
+++ b/library/cpp/protobuf/json/proto2json_printer.cpp
@@ -253,7 +253,7 @@ namespace NProtobufJson {
                                               bool inProtoMap) {
         Y_ABORT_UNLESS(!field.is_repeated(), "field is repeated.");
 
-        if (!key) {
+        if (!key && !inProtoMap) {
             key = MakeKey(field);
         }