Cpp: Handle empty recipients consistently
* lang/cpp/src/context.cpp (Context::getKeysFromRecipients): New helper. (Context::encrypt, Context::startEncryption, Context::signAndEncrypt) (Context::startCombinedSigningAndEncryption): Use new helper. * lang/cpp/src/context.h (Context::getKeysFromRecipients): Add as private helper.
0f66586 was incomplete as the code to handle recpients
was duplicated four times. This is now unified and constently
uses a nullptr instead of an empty array.