fix(dataobj): Handle constant predicates in logs predicate ordering

[paul1r]

What this PR does / why we need it:

DataObjScan can map engine filters to dataset.TruePredicate and
dataset.FalsePredicate, but getPredicateSelectivity panicked when
orderPredicates scored them. Return fixed selectivity for constants so
impossible filters no longer crash queriers during reader init.
Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

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• Documentation added
• Tests updated
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