Low: scheduler: avoid notification problems if node names are unknown

Previously, expand_node_list() would set the node name lists to a single space
if the node list were NULL. However, if the node name was unknown for all nodes
in the list, the node name lists would end up NULL.

Fix this, along with best practices for the function: rename it to
get_node_names() for clarity, add a doxygen block, rename variables,
use the output arguments directly to avoid needing a couple variables,
use convenience functions where appropriate, and improve comments and
formatting.