find_package: Allow cmDirectoryListGenerator::Names to be nullptr

Change cmDirectoryListGenerator::Names to be a pointer rather than a
reference wrapper. This allows it to be a null pointer, which allows
cmAnyDirectoryListGenerator to pass a nullptr rather than needing to
construct and hold an empty list just to satisfy the reference being
non-null. Importantly, it also make it more obvious that anyone
constructing a cmDirectoryListGenerator or subcless thereof needs to
pass something that isn't going to immediately go out of scope.