Handle empty translation requests

Fixes https://github.com/browsermt/bergamot-translator/issues/101.
ResponseBuilder is called with empty histories to trigger a valid but
mostly-empty response.

src/translator/request.cpp

@@ -20,6 +20,13 @@ Request::Request(size_t Id, Segments &&segments,
 
   counter_ = segments_.size();
   histories_.resize(segments_.size(), nullptr);
+
+  // If there are no segments_, we are never able to trigger the responseBuilder
+  // calls from a different thread. However, in this case we want an empty valid
+  // response.
+  if (segments_.size() == 0) {
+    responseBuilder_(std::move(histories_));
+  }
 }
 
 size_t Request::numSegments() const { return segments_.size(); }