CASE WHEN ... THEN ... END without an ELSE branch returns NULL for
unmatched rows. Often unintended - the author thought the WHEN
conditions were exhaustive but they aren't, or the downstream code
can't handle NULL.

Should fail

SELECT CASE
  WHEN status = 'paid' THEN 1
  WHEN status = 'pending' THEN 0
END AS paid_flag
FROM orders;

Should pass

SELECT CASE
  WHEN status = 'paid' THEN 1
  WHEN status = 'pending' THEN 0
  ELSE NULL
END AS paid_flag
FROM orders;

Implementation hints

• Multi-line rule.
• Detect CASE ... END without ELSE between.
• Test with nested CASE expressions - make sure outer CASE without ELSE
  still triggers when inner CASE has ELSE.
• Severity: warning.

Estimated LOC: ~35 code + ~25 test.