Refactor conditional function tests into parametrized test suite

Replace separate test functions for coalesce, greatest, least, nvl,
nvl2, ifnull with a single parametrized test using a shared fixture.
Adds coverage for nvl, nullif (previously untested), datetime and
boolean types, literal fallbacks, and variadic calls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer