関連ワード:  java test list is null or empty    java check if list is null or empty    java check if collection is null or empty    java 8 check if list is null or empty using optional    java check if list is not null or empty    java 8 check if list is null or empty before stream    java 17 check if list is null or empty    java 8 check if list is null or empty    java util check if list is null or empty    java check if list object is null or empty