refactor: extract magic number 0L to constant for new entity check

[currenjin]

Motivation

Replaced the hardcoded value 0L with a named constant to clarify its meaning as a marker for new (unsaved) entities.

Even though the value was only used in a single location, extracting it improves code readability and prevents potential misuse.

Changes

• Introduced a private static final constant NEW_ID = 0L
• Replaced inline 0L usage in new entity check

Checklists

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• You submit test cases (unit or integration tests) that back your changes.
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[mp911de]

Thank you for your contribution. I'm not quite sure this is appropriate. We would have to add another constant for null if the identifier is an object type and not an primitive to reflect the entire story. Introducing constants isn't helpful as it doesn't help in explaining the concept. Also, we have other places (like IsNewStrategy) that follow a similar behavior.

Therefore, we do not want to merge this PR.