Why does returning null for a primitive work in this case?

The first has a ternary operator which has a result type of Boolean. The NPE is converting a null to a boolean.

It is actually something like:

Boolean temp = s != null ? s.isEmpty() : null; //no problems here
return temp; //crash when temp==null

The second is trying to return a wrong type (Object instead of primitive) - and thus does not compile.