Handle empty node_ids in importcontent

If node_ids is set to an empty string, read it as an empty list,
instead of a list with a single empty string.

kolibri/core/content/management/commands/importcontent.py

@@ -92,7 +92,7 @@ def add_arguments(self, parser):
             "--node_ids",
             "-n",
             # Split the comma separated string we get, into a list of strings
-            type=lambda x: x.split(","),
+            type=lambda x: x.split(",") if x else [],
             default=None,
             required=False,
             dest="node_ids",
@@ -109,7 +109,7 @@ def add_arguments(self, parser):
         parser.add_argument(
             "--exclude_node_ids",
             # Split the comma separated string we get, into a list of string
-            type=lambda x: x.split(","),
+            type=lambda x: x.split(",") if x else [],
             default=None,
             required=False,
             dest="exclude_node_ids",