Don't allow registration of actions for undefined nodes
Fixed bug where giant tree ferns dropped unknown node

gianttreefern.lua depended upon a node (ferns:tree_fern_leaves) that was defined in fern.lua. If fern.lua was never loaded (i.e. Tree_Fern set to false in the settings file) then giant treeferns would drop an unknown node.
Merge branch 'Zeno' into mtz
Bugfix: giant tree fern leaves dropping wrong item
Merge branch 'Zeno' into mtz