Return devicenodes as empty list if no LUN's

got connected. It is possible for an intiator to successfully connect to a
target, whilst getting no LUN's back. If no devicenodes get detected, it makes
more sense to return an empty list than plainly None.

This potentially avoids further tasks to have to check if devicenodes is
iterable.

system/open_iscsi.py

@@ -206,9 +206,6 @@ def target_device_node(module, target):
     # a given target...
 
     devices = glob.glob('/dev/disk/by-path/*%s*' % target)
-    if len(devices) == 0:
-        return None
-    else:
     devdisks = []
     for dev in devices:
         # exclude partitions