Filter empty values in Smt::with_entries

[PhilippGackstatter]

Describe your changes

The partial_smt_root_mismatch_on_empty_values test passes key-value pairs to Smt::with_entries where some of the values are EMPTY_WORD. This causes a panic in the concurrent subtree computation of the Smt on current next, e.g.: https://github.com/0xPolygonMiden/crypto/actions/runs/13387187735/job/37386455081#step:4:500.

One fix is to make the test not pass empty values, which is actually unnecessary, since it's the default value. However, I think with_entries shouldn't panic when empty values are passed, so I fixed that instead by filtering them out early in the process.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[bobbinth]

Looks good! Thank you!