By default, records with an empty-string value in a join field are
paired just like any other value, so two records both missing an ID
get matched with each other -- rarely the intended behavior.
--ignore-empty treats an empty-string join-field value as if the
field were absent, on both the left and right files, so such records
fall through to unpaired handling (--np/--ul/--ur) instead of
cross-joining on the empty string.
Wires the check through both the default half-streaming join and the
-s/--sorted-input doubly-streaming join, which track left-file
buckets independently and needed the same empty-aware key-presence
check in JoinBucketKeeper.
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
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