When a new mapblock is generated and the mod checks the neighbors around
a target to place a fallen twig, if it finds an unknown node (because
it's in a neighboring, old mapblock from a previous session -- perhaps
an old moss node that hadn't converted-over to wallmounted yet), trying
to check its buildable_to state will fail, since that requires that
there be a node def to look at, which an unknown node wouldn't have.
This substitutes a known not-buildable_to node for those cases, so that
the code won't try to overwrite what it found.
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