message: Added `alwaysIncludeEmptyCollectionElements` to `Serializer`
diff: @@ -74,6 +74,11 @@ public:
  */
  bool alwaysIncludeRequired = false;
  + /**
  + * Always include array or map elements, even if their value is `nullptr`.
  + */
  + bool alwaysIncludeEmptyCollectionElements = false;
  +
  /**
  * If `true` - insert string `"<unknown-type>"` in json field value in case unknown field found.
  * Fail if `false`.
  @@ -136,7 +141,7 @@ private:
  bool first = true;
  for(auto& value : *list) {
  - if(value || serializer->getConfig()->includeNullFields) {
  + if(value || serializer->getConfig()->includeNullFields || serializer->getConfig()->alwaysIncludeEmptyCollectionElements) {
  (first) ? first = false : stream->writeSimple(",", 1);
  serializer->serialize(stream, value);
  }
  @@ -161,7 +166,7 @@ private:
  for(auto& pair : *map) {
  const auto& value = pair.second;
  - if(value || serializer->getConfig()->includeNullFields) {
  + if(value || serializer->getConfig()->includeNullFields || serializer->getConfig()->alwaysIncludeEmptyCollectionElements) {
  (first) ? first = false : stream->writeSimple(",", 1);
  const auto& key = pair.first;
  serializeString(stream, key->getData(), key->getSize());
  