Robustness check for system disk entities.

When the system information is still loading it may happen that the
device associated to the disk hasn't been loaded yet.

platypush/plugins/system/__init__.py

@@ -606,6 +606,7 @@ class SystemPlugin(SensorPlugin, EntityManager):
                     **disk,
                 )
                 for disk in entities['disks']
+                if disk.get('device')
             ],
             *[
                 NetworkInterfaceModel(