When a nullable scalar field has no inverted index, the forward filter path
fails to handle null values from Arrow's filter evaluation:
1. get_forward_bit(): BooleanArray::operator[] returns nullopt for null entries,
which is_filtered() treats as "no filter" (not filtered), letting null docs
through. Fix: use value_or(false) to treat null as "not matched".
2. is_matched_by_forward_filter(): reads BooleanScalar.value without checking
is_valid, which is UB for null scalars. Fix: check is_valid first.
Closes#409
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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