Check for missing node in uuid_node_features_export_render()

When enabling a uuid_node feature using drush --verbose, I get the following warning: "Trying to get property of non-object uuid_node.features.inc"

This is because the code which puts the date into the format expected by node_object_prepare() doesn't check that node_get_by_uuid() actually returned anything other than an empty array. This check fixes the warning.