Use isEmpty() instead of length()

[dukbong]

This pull request suggests using isEmpty() instead of length().

Rationale:

• Readability: The isEmpty() method clearly conveys the intention of checking if the collection is empty. Therefore, it makes it easier for developers to understand the code.
• Intended Behavior: The isEmpty() method is designed to check if a collection is empty. This aligns better with the developer's intention and makes the code more expressive.
• Common Convention: In the Java collection framework, it's common to use the isEmpty() method to check if a collection is empty. Following this common convention helps maintain consistency in the codebase.

In conclusion, using the isEmpty() method improves code readability and aligns better with the intended behavior. It also adheres to the common convention in the Java collection framework.

[coveralls]

coverage: 87.167%. remained the same when pulling a020b39a078975b0181dfdccef93cd1a5c6efa54 on dukbong:Refactoring into 382cbf4da448ffdc1a87221112482bc4f0894b5d on mybatis:master.