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
"""

Author: PJHsieh

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):
         """

tests/component_base_test.py

@@ -0,0 +1,7 @@
+from sonic_platform_base.component_base import ComponentBase
+
+class TestComponentBase:
+
+    def test_get_firmware_update_notification(self):
+        cpnt = ComponentBase()
+        assert(cpnt.get_firmware_update_notification(None) == "None")