libbinder_ndk: consider empty result as false

associateClassInternal returns std::optional<bool> where nullopt means
not sure yet, true means success, and false means failure. However,
there was one case where we considered nullopt to mean true.

Bug: N/A
Test: atest aidl_integration_test
Change-Id: I7caef23dfbd8d9a562a21a0f369fa22c976a8da0