refactor: replace size checks with isEmpty()

[arturhg]

Refactor conditional statements across various classes to use isEmpty() instead of size checks for better readability and performance.

• Changed Edition.java to use isEmpty() for checking pageData.
• Changed GradeCopyGradeScaleDialog.java to use isEmpty() for gradeElements.
• Changed GradeTab.java to use isEmpty() for checking parent.getChildren().
• Changed GradeExportRenderer.java to use isEmpty() for file.comments.
• Changed ListsManager.java to use isEmpty() for TextTreeSection.lists.