[Component]Fix test_get_firmware_update_notification fail

Erro Log:

> pytest_assert(isinstance(notif, STRING_TYPE), "Component {}: Firmware update notification appears to be incorrect from image {}".format(i, image))
E Failed: Component 0: Firmware update notification appears to be incorrect from image current

Refer to API get_firmware_update_notification header, shall return
'None' instead of None.

"""
Retrieves a notification on what should be done in order to complete
the component firmware update

Args:
    image_path: A string, path to firmware image

Returns:
    A string containing the component firmware update notification if required.
    By default 'None' value will be used, which indicates that no actions are required
"""

sonic_platform_base/component_base.py

@@ -76,7 +76,7 @@ def get_firmware_update_notification(self, image_path):
             A string containing the component firmware update notification if required.
             By default 'None' value will be used, which indicates that no actions are required
         """
-        return None
+        return 'None'
 
     def install_firmware(self, image_path):
         """