remove optionality of field dictionary

[agavra]

fixes #25

[github-actions]

📊 Benchmark Comparison Report

This pull request includes Criterion benchmarks comparing performance to the main branch.
This comment will automatically update as the benchmarks are re-run on each commit.

The table below shows relative ratios and timing stats for each benchmark group:

group                main                                   pr
-----                ----                                   --
ops/merge            1.00  1428.3±26.27ns        ? ?/sec    1.10  1573.3±16.55ns        ? ?/sec
ops/project          1.00    586.3±7.97ns        ? ?/sec    1.02    598.3±7.19ns        ? ?/sec
serde/deserialize    1.00    271.2±3.60ns        ? ?/sec    1.03    279.2±1.93ns        ? ?/sec
serde/serialize      1.08    276.1±7.45ns        ? ?/sec    1.00   256.5±20.00ns        ? ?/sec

✅ Benchmarks completed successfully.

🧠 Notes:

• These benchmarks are not a pass/fail gate and are informative only.
• Use this as a signal to review performance-sensitive changes.
• Results may be unreliable due to GHA runner hardware variance.
• If results indicate a significant performance regression, run the benchmarks locally to confirm.

Reported by the benchmark CI bot

[cadonna]

Thanks for the PR @agavra !

LGTM!