message: Avoid memcpy call for empty vectors
diff: @@ -1607,6 +1607,9 @@ class FlatBufferBuilder {
  // causing the wrong overload to be selected, remove it.
  AssertScalarT<T>();
  StartVector(len, sizeof(T));
  + if (len == 0) {
  + return Offset<Vector<T>>(EndVector(len));
  + }
  // clang-format off
  #if FLATBUFFERS_LITTLEENDIAN
  PushBytes(reinterpret_cast<const uint8_t *>(v), len * sizeof(T));
  