関連ワード:  java list check if null or empty    java list check for null or empty    java check list is not null or empty    java 8 check list 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 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