The while condition should add carry != 0, like this:

while( l1 != null || l2 != null || carry ==1){
}

otherwise, it cannot pass the test case like:
[9,9,9,9,9,9,9]
[9,9,9,9]

output should be:
[8,9,9,9,0,0,0,1]
but it missed the highest [1]

thx