fix: 검증 로직 일부 수정

main/src/main/java/org/sopt/makers/crew/main/meeting/v2/service/MeetingV2ServiceImpl.java

@@ -543,7 +543,7 @@ private void validateApplyPeriod(Meeting meeting) {
 	}
 
 	private void validateUserActivities(User user) {
-		if (user.getActivities().isEmpty()) {
+		if (user.getActivities() == null || user.getActivities().isEmpty()) {
 			throw new BadRequestException(MISSING_GENERATION_PART.getErrorCode());
 		}
 	}