SessionManager: Change behavior of getSessionById()
It's easily possible for SessionManager::getSessionById() to not be
able to load the specified session and to not be able to create an empty
one by that ID, for example if the user's token changed. So change this
from an exceptional condition to an expected one, and adjust callers to
deal with it appropriately.
Let's also make the checks for invalid data structure when loading the
session from the store delete the bogus data entirely.
At the same time, let's change the silly "$noEmpty" parameter to
"$create" and make the default behavior be not to create an empty