CONTRIBUTING: added positive/negative test cases requirement

CONTRIBUTING.md

@@ -7,6 +7,7 @@ Thank you for investing your time and effort in contributing to our project, we
 - Keep the code compatible with Java 17.
 - Avoid adding new dependencies as much as possible (new dependencies with test scope are OK). If absolutely necessary, try to use the same libraries which are already used in the project. Make sure you run `mvn dependency:analyze` to identify unnecessary dependencies.
 - Write unit and/or integration tests for your code. This is critical: no tests, no review!
+- The tests should cover both positive and negative cases.
 - Make sure you run all unit tests on all modules with `mvn clean test`
 - Avoid making breaking changes. Always keep backward compatibility in mind. For example, instead of removing fields/methods/etc, mark them `@Deprecated` and make sure they still work as before.
 - Follow existing naming conventions.