compiler: fix unevaluation of sparse cluster

[mloubout]

Fix issue with pickling operator with sparse + subs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.99%. Comparing base (97c3526) to head (03eb51b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2435 +/- ## ======================================= Coverage 86.99% 86.99% ======================================= Files 236 236 Lines 44837 44839 +2 Branches 8361 8362 +1 ======================================= + Hits 39007 39009 +2 Misses 5101 5101 Partials 729 729 ```

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[FabioLuporini]

Thanks, merged