Make ELSE optional in CASE expressions

[sieun0322]

Resolves #4142

Summary

Makes the ELSE clause optional in CASE expressions

Changes

• Updated the ANTLR grammar files for both JPQL and EQL parsers to make the ELSE clause optional by wrapping (ELSE scalar_expression) with ? quantifier.
• Added test cases for optional ELSE scenarios

Checklist

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.

For future contributions, please avoid excessive summarization that is typical for AI tools, a short and concise description (such as Updated the JPQL and EQL ANTLR grammar to make the ELSE clause optional) is fully sufficient.