Fix another minor YAML bug

- using list related functionality on non-lists or non-existent keys
would produce odd results and errors, as empty lists weren't compensated
for in any way. Now they're handled appropriately.
Prevent null-NPCs from respawning

- if an NPC died, then was removed... it would respawn anyway, creating
a lot of weird issues. Will now ensure the NPC is still valid before
respawning it.
Merge branch 'master' of https://github.com/aufdemrand/Denizen
Fix 'on remove assignment'

The action would never fire because it was run /after/ the assignment
was removed rather than before. The action will now fire on the current
assign script before removing it entirely.
Merge pull request #728 from mcmonkey4eva/master

Pull request for weird minor issue fixes