PackageInfo::queryDrvPath(): Don't dereference an empty optional

Fixes a regression introduced in f923ed6b6a.

https://hydra.nixos.org/build/262267313

src/libexpr/get-drvs.cc

@@ -82,8 +82,7 @@ std::optional<StorePath> PackageInfo::queryDrvPath() const
         } else
             drvPath = {std::nullopt};
     }
-    drvPath.value_or(std::nullopt);
-    return *drvPath;
+    return drvPath.value_or(std::nullopt);
 }