core: NodeApplyableResourceInstance.References returns its result

A trivial mistake in the rework of this function meant that it was just
discarding its result rather than returning it. It will now return its
result as expected, allowing reference analysis to work for this node
type.

terraform/node_resource_apply.go

@@ -64,7 +64,7 @@ func (n *NodeApplyableResourceInstance) References() []*addrs.Reference {
 		}
 	}
 
-	return nil
+	return ret
 }
 
 // GraphNodeEvalable