Support empty node_ids in get_import_export_nodes

When node_ids and exclude_node_ids are empty lists, we can understand
this to mean that no nodes are being selected. This is distinct from
when they are None, which means the default behaviour of selecting all
nodes. This is useful to provide a sane fallback when a manifest file
has no information about a channel.

kolibri/core/content/utils/import_export_content.py

@@ -174,7 +174,7 @@ def get_import_export_nodes(  # noqa: C901
         nodes_query = nodes_to_include
 
         # if requested, filter down to only include particular topics/nodes
-        if node_ids:
+        if node_ids is not None:
             nodes_query = nodes_query.filter_by_uuids(
                 _mptt_descendant_ids(
                     channel_id, node_ids, min_boundary, min_boundary + dynamic_chunksize
@@ -186,7 +186,7 @@ def get_import_export_nodes(  # noqa: C901
             nodes_query = nodes_query.filter(renderable_contentnodes_q_filter)
 
         # filter down the query to remove files associated with nodes we've specifically been asked to exclude
-        if exclude_node_ids:
+        if exclude_node_ids is not None:
             nodes_query = nodes_query.order_by().exclude_by_uuids(
                 _mptt_descendant_ids(
                     channel_id,