Make it hard to construct an empty OutputsSpec::Names

This should be a non-empty set, and so we don't want people doing this
by accident. We remove the zero-0 constructor with a little inheritance
trickery.

src/libstore/path-with-outputs.cc

@@ -53,7 +53,7 @@ std::variant<StorePathWithOutputs, StorePath> StorePathWithOutputs::tryFromDeriv
                         return {};
                     },
                     [&](const OutputsSpec::Names & outputs) {
-                        return outputs;
+                        return static_cast<StringSet>(outputs);
                     },
                 }, bfd.outputs.raw()),
             };