More robust handling of empty bootacl sysfs/database file

BoltStore::domain_get will store the BoltDomain without bootacl support, i.e. bootacl == NULL as "bootacl=". Loading that file via BoltStore::domain_get that would lead to an empty bootacl, i.e. bootacl == {NULL} object property. This lead to some internal issues, where e.g. bolt_domain_supports_bootacl would return TRUE because it only checked for NULL (but bootacl was{NULL}). In theory a test case was there to check for that, but the test was broken :/

Fix all this, i.e. make BoltStore::domain_get return NULL, but also treat {NULL} and NULL for bootacl in the same way where that is semantically true; obviously fix the test and add some more (hopefully correct ones).