chore: add NULLIF test case

[tianzhou]

Validate #103

[Copilot]

Pull Request Overview

This PR adds a new test case for validating NULLIF, GREATEST, and LEAST SQL functions, specifically targeting issue #103 related to divide-by-zero scenarios. The test case verifies that the schema diff tool correctly handles views containing these functions.

Key Changes:

• Added a new view nullif_functions_view demonstrating NULLIF usage for divide-by-zero protection
• Includes test coverage for GREATEST, LEAST, and complex CASE expressions with NULLIF
• Updated test plan files to reflect the addition of the new view

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/diff/create_view/add_view/plan.txt	Updated expected plan output to include the new nullif_functions_view
testdata/diff/create_view/add_view/plan.sql	Added SQL DDL for the new view to the expected plan SQL
testdata/diff/create_view/add_view/plan.json	Added JSON representation of the create operation for the new view
testdata/diff/create_view/add_view/new.sql	Added the source SQL definition for nullif_functions_view with comprehensive comments
testdata/diff/create_view/add_view/diff.sql	Added the new view to the expected diff output

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