Create Maya USD: Do not error on context attribute value change

[BigRoy]

Changelog Description

Create Maya USD: Do not error on context attribute value change

Additional review information

Should fix error posted here

Works nicely, the only occasion which throw some traceback is when trying to switch settings for context and Validate outdated containers

resulting in

// Traceback (most recent call last):
//   File "C:\Work\REPO\ayon-core\client\ayon_core\lib\events.py", line 332, in process_event
//     callback(event)
//   File "c:\Work\REPO\ayon-maya\client\ayon_maya\plugins\create\create_maya_usd.py", line 30, in on_values_changed
//     if instance["creator_identifier"] != self.identifier:
// TypeError: 'NoneType' object is not subscriptable

Not failing but worth mentioning imho....

Testing notes:

1. Create maya usd instance
2. Trigger context attribute change
3. No error should occur