test: use jq // empty instead of grep -v null in Unit.List tests

test: use jq // empty instead of grep -v null in Unit.List tests

Replace `grep -v null` with jq's `// empty` alternative operator when filtering unit IDs. With `set -o pipefail`, `grep` returns 1 when no lines match, which aborts the script before conditional guards can run. The `// empty` operator suppresses null output directly in jq without risking a pipeline failure.