Better handle daemon call creation failure
Even if the daemon fails to create a call, the new LRC's call model still creates an invalid call entry and adds it to pimpl_->calls
.
This is a bug, in this case we should return an empty string and leave the map untouched.
https://git.ring.cx/savoirfairelinux/ring-lrc/blob/master/src/newcallmodel.cpp#L178