fix toString in the Java backend

Use Objects.toString(obj) to handle null

Bug: n/a
Bug: 150161123
Test: aidl_integration_test
Merged-In: I8bcc7346e4568df1e5371268a0acd9e14f386805
Change-Id: I8bcc7346e4568df1e5371268a0acd9e14f386805
(cherry picked from commit ee73cb33fb727f97ff39af6e8fcba7caa4156520)
1 file changed