libbinder_rs: Handle empty args list for dump transactions

Previously we expected the args pointer passed to the handler for dump
transactions to be non-null. If there are no arguments this
pointer may be null and we should pass an empty args list instead of
erroring out.

Bug: 232887857
Test: atest -p frameworks/native/libs/binder
Test: cts -m  CtsSecurityTestCases -t android.security.cts.ServicePermissionsTest#testDumpProtected
Change-Id: If2b08c688a374913ef0913749efc72db07e7776f