Fix generic access to empty lists
When reading json files into boost::ptree a shortcoming of ptree becomes obvious. The standard ptree, which is conceptually a
struct ptree
{
string data; // data associated with the node
list< pair<string, ptree> > children; // ordered list of named children
};, cannot distinguish between an empty string entry key -> "" and an empty array entry key -> []. In both cases the internal data string and the children list are empty.
This was noticed and mentioned by @aporeba in MR !30690 (merged). Here I am fixing this issue by removing the exception throwing for this particular case. If getList(key) is called for key -> "", the return value will be and empty vector<TrigConf::DataStructure>