fix(consistency-checker): guard empty denominators

[Abelo9996]

What

Guard two empty-denominator divisions in the consistency checker that raise a ZeroDivisionError on a failed run. The exception propagates out of ConsistencyChecker.run(), and the CLI catches it per journal, so the tool does not crash, but that journal's entire consistency check is abandoned and reported as an opaque Error running consistency check float division by zero instead of the actual findings.

Why

The consistency checker exists to flag broken runs, but two checks crash on exactly those runs:

• MinValidAnnotatorItems.check divides by num_translated_annotations, which is 0 when an annotator produced no translated annotations (for example, its API was down for the whole run). run_checks_for_row calls check.check() with no guard, and the runner iterates every annotator the test declares, so the exception propagates out of ConsistencyChecker.run(); the CLI's run_consistency_check catches it per journal, abandoning that journal's whole check and reporting a generic error instead of the specific finding.
• HazardScoreIsFracSafe._get_frac_safe divides by num_scored, which is 0 when no items were measured for a hazard; this runs in __init__, so constructing the check crashes.

Details and a standalone repro are in #1629.

Change

• MinValidAnnotatorItems.check returns False (with a clear failure message) when there are no translated annotations, flagging the annotator instead of crashing.
• HazardScoreIsFracSafe._get_frac_safe returns nan when nothing was measured, so the equality check fails rather than crashing.
• Regression tests for both.

This mirrors the empty-denominator guard already in flight in #1608.

Fixes #1629

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[wpietri]

Thanks for taking an interest, @Abelo9996. Is this a problem you ran into while using ModelBench? If so, I'd like to know more about the circumstances. Thanks!