Allow digging of unknown nodes

Allow digging of unknown nodes

This allows the removal of nodes with unknown types.
get_item_callback() (C++) would fail if a node has an unknown type.  Now it
will try using the callback from minetest.nodedef_default in this case.
Also, minetest.node_dig() (Lua) was altered to always allow digging when
the node definition is empty (i.e. unknown node).