関連ワード:  java list is null or empty check    java check if list is null or empty    java 8 check list is null or empty    java 8 check if list is null or empty using optional    java check list is not null or empty    java 17 check if list is null or empty    java util check if list is null or empty    java 8 check if list is null or empty    java 21 check if list is null or empty    java 8 check if list is null or empty before stream