Add TODO about setting null instead of empty string in JSON

An empty string is a sneaky way to avoid hard failures --- things that
expect strings still get strings, but it does conversely open the door
up to soft failures (spooky-action-at-a-distance ones because the string
did not have the expected invariants).

"Fail fast" with null will ultimately make the system more robust, but
force us to fix more things up front, and I don't want to change this
without also fixing those things up front, especially as this commit is
for now just part of the the preparatory PR for which this is dead code.

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

@@ -250,6 +250,10 @@ 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] = "";
                     }
                 }