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