* lang/cpp/src/context.cpp
  (Context::signaturePolicyURL): return nullptr on default
  (to_tofu_policy_t): add default case for unknown
* lang/cpp/src/key.cpp
  (Key::primaryFingerprint): return nullptr on default
* lang/cpp/src/tofuinfo.cpp
  (GpgME::TofuInfo::policy): add default case for unknown