Fix node replacement in not-quite-loaded chunks

When first entering an area, sometimes placing nodes replaces other
nodes that are not buildable_to. This seems to be caused by the fact
that nodes in unloaded map blocks are treated as ignore, a node that is
buildable_to. This fixes that, by using get_node_or_nil() instead of the
previously-used get_node(), then checking to see if the nodes were
actually loaded before replacing.