refactor: Improve exists check in JpaQueryExecution

[minwoo1999]

Changes

Updated getResultList().isEmpty() to getResultStream().findFirst().isPresent() for checking query existence.
Improved performance by processing only the first result.

Reason

It seems that this method is only meant to check if the query exists. Therefore, fetching the entire query result does not seem necessary.
Avoids loading all results into memory, reducing overhead and improving efficiency.

Testing

Verified with existing test cases, ensuring identical results.

Review Request

Please review and share any feedback. 🙇‍♂️

[mp911de]

ExistsExecution is only used for derived queries where we set query.setMaxResults(1). Therefore, instantiating a Java 8 stream instead of reading one result into a List is almost the same overhead. We want to keep our code as-is.