Eval cache: fix cache regressions

- Fix eval cache not being persisted in `nix develop` (since #10570)
- Don't attempt to commit cache transaction if there is no active transaction, which will spew errors in edge cases
- Drive-by: trivial typo fix

src/libexpr/eval-cache.cc

@@ -95,7 +95,7 @@ struct AttrDb
     {
         try {
             auto state(_state->lock());
-            if (!failed)
+            if (!failed && state->txn->active)
                 state->txn->commit();
             state->txn.reset();
         } catch (...) {