関連ワード:  java list check if empty or null    java check list is empty or null    java check if list is not empty or null    java 8 check if list is null or empty using optional    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    java check if list object is null or empty