Add condition to check whether the user has requested a change to the...

Add condition to check whether the user has requested a change to the proctoring_provider Advanced Setting. In the case that the setting is not available (i.e. the ENABLE_PROCTORING_PROVIDER_OVERRIDES waffle flag is not enabled for the course), the requested proctoring_provider will be None. We should consider, in this case, that the user has not requested a change to the proctoring_provider setting and therefore should not prevent the user from saving course settings if the other conditions for preventing changes to the proctoring_provider setting are met.