You can use the CONNECT_BY_ISLEAF
pseudocolumn, which returns 1 if current node is a leaf, and 0 otherwise, so your query should be rewritten like this:
SELECT A.PRODUCTCATEGORYID, A.PARENTCATEGORYID, SYS_CONNECT_BY_PATH(A.LABEL, ':') "PATH",
CONNECT_BY_ISLEAF as "LEAF"
FROM PRODUCT_CATEGORY A
CONNECT BY PRIOR A.PRODUCTCATEGORYID = A.PARENTCATEGORYID
START WITH A.PARENTCATEGORYID IS NULL;
Read more in Oracle's documentation: CONNECT_BY_ISLEAF pseudocolumn