More robustness for the spin viewer GUI window prune_*() methods.
When no spin data exists, the self.tree.GetItemPyData(key) call can return None. This is now being
checked for and such None values are being skipped in the prune_mol(), prune_res() and prune_spin()
methods.
The problem was found in the Mf.test_bug_20479_gui_final_pipe system test when running the command:
for i in {1..10}; do ./relax --gui-tests --time -d &>> gui_tests.log; done
