Find all unique columns that are allowed null
Try this one:

SELECT ind.OWNER AS INDEX_OWNER, ind.INDEX_NAME,
    tab.OWNER AS TABLE_OWNER, tab.table_name,
    tab.column_Name,
    UNIQUENESS, NULLABLE
FROM ALL_INDEXES ind
    JOIN ALL_IND_COLUMNS col 
        ON ind.OWNER = INDEX_OWNER 
            AND ind.INDEX_NAME = col.INDEX_NAME
    JOIN ALL_TAB_COLUMNS tab 
        ON tab.owner = col.table_owner 
            AND tab.table_name = col.table_name 
            AND tab.column_Name = col.column_Name
WHERE NULLABLE='Y' AND UNIQUENESS='UNIQUE';