chore(noir): simplify use of get_commitment in prep for larger PR

[dbanks12]

Description

• also a fix to computation of nullified commitments in noir
• main task after this prep: getCommitment should be able to work with pending commitments #1029
• another follow up task Spike: replace getCommitments oracle with checkCommitmentExists? #1142

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• The branch has been merged or rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.

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[ludamad]

Closing as stale. Reopen as needed.