Fix failed register null listener tests

aosp/1574060 changed the binder_to_string format which would
make register null listener tests failed on Android S. Because
the log patterns are different between dumpsys and the tests.
However, these tests must pass on Android Q/R which are still
using the old format. Thus, to be compatible from all version,
just remove the "(null)" argument from output logs.

Bug: 179474903
Test: atest DnsResolverBinderTest on R & S build.
Change-Id: I1f1c93c96c9e5c51d55cc378a04aa8ebb0526e10