Default to key_t() instead of "" for empty_config
Default to key_t() instead of "" for empty_config

Kitware/sprokit,mathstuf/sprokit,mathstuf/sprokit,Kitware/sprokit,linus-sherrill/sprokit,linus-sherrill/sprokit,linus-sherrill/sprokit,mathstuf/sprokit,Kitware/sprokit,mathstuf/sprokit,linus-sherrill/sprokit,Kitware/sprokit