Support IsEmpty and IsNotEmpty keywords in derived queries

[backend-choijunhyeong]

Add support for IsEmpty and IsNotEmpty repository query keywords in derived queries.

Why

The documentation lists IsEmpty as a supported keyword, but it throws
"Unsupported keyword" exception when used.

Changes

• Add IS_EMPTY and IS_NOT_EMPTY case handling in MongoQueryCreator.from()
• Add createIsEmptyCriteria() and createIsNotEmptyCriteria() methods
• String properties: { field: { $eq: "" } } / { field: { $ne: "" } }
• Collection properties: { field: { $size: 0 } } / { field: { $not: { $size: 0 } } }

Resolves #4606

[backend-choijunhyeong]

I'd like to clarify the expected behavior regarding null values.

Current implementation

• IsEmpty: matches only "" (empty string) or size: 0 (empty collection)
• IsNotEmpty: matches anything that is not "" or size: 0

This means null values are NOT considered "empty".

Question
Should IsEmpty also match null values?
For example: findByFirstNameIsEmpty -> should it return documents where firstName is null?

Options

1. Current: null is NOT considered empty (strict interpretation)
2. Alternative: null OR "" is considered empty (lenient interpretation)

I can adjust the implementation based on the team's preference.