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