More CA derivations prep

Again, with care not to change the schema in any way.

src/hydra-eval-jobs/hydra-eval-jobs.cc

@@ -250,11 +250,7 @@ static void worker(
                         // See the `queryOutputs` call above; we should
                         // not encounter missing output paths otherwise.
                         assert(experimentalFeatureSettings.isEnabled(Xp::CaDerivations));
-                        // TODO it would be better to set `null` than an
-                        // empty string here, to force the consumer of
-                        // this JSON to more explicitly handle this
-                        // case.
-                        out[outputName] = "";
+                        out[outputName] = nullptr;
                     }
                 }
                 job["outputs"] = std::move(out);