fix: cursorId가 'null'인 경우 500에러가 뜨는 버그 수정

lime-domain/src/main/java/com/programmers/lime/domains/item/repository/ItemRepositoryForCursorImpl.java

@@ -193,7 +193,7 @@ private BooleanExpression cursorIdCondition(
 		final String cursorId,
 		final ItemSortCondition itemSortCondition
 	) {
-		if (cursorId == null) {
+		if (cursorId == null || cursorId.equals("null")) {
 			return null;
 		}