Raid controller and Physical Drive attributes need to be nullable

This can fail with
AttributeError: 'NoneType' object has no attribute 'strip'
otherwise

Closes-Bug: #1550328
Change-Id: I8eafe606407cddc1e9ea03a33473b65ff3b59384

dracclient/resources/raid.py

@@ -187,7 +187,8 @@ def _parse_drac_raid_controller(self, drac_controller):
 
     def _get_raid_controller_attr(self, drac_controller, attr_name):
         return utils.get_wsman_resource_attr(
-            drac_controller, uris.DCIM_ControllerView, attr_name)
+            drac_controller, uris.DCIM_ControllerView, attr_name,
+            nullable=True)
 
     def list_virtual_disks(self):
         """Returns the list of virtual disks
@@ -290,7 +291,7 @@ def _parse_drac_physical_disk(self, drac_disk):
 
     def _get_physical_disk_attr(self, drac_disk, attr_name):
         return utils.get_wsman_resource_attr(
-            drac_disk, uris.DCIM_PhysicalDiskView, attr_name)
+            drac_disk, uris.DCIM_PhysicalDiskView, attr_name, nullable=True)
 
     def convert_physical_disks(self, physical_disks, raid_enable):
         """Converts a list of physical disks into or out of RAID mode.