NPE Fix for when player joins and there is no cell.

The cell will always be null whenever the jail contains no cells, so do
another check for null and if it isn't null then teleport them to the
cell but if it is null then just teleport them into the jail's teleport
location.
Merge pull request #2 from graywolf336/364_NPE_Fix

NPE Fix for when player joins and there is no cell. - Thanks graywolf336