Revert system.get_node_message() and system.get_telegram() to return null

... upon failure/no message waiting. Some scripts (e.g. ecWebv4) explicitly
check for a null return value. So document the return value will be null when
there's no message waiting.
This isue was introduced in commit f300a32e and reported via
DOVE-Net by kk4qbn (KK4QBN)