Remove NSNull as a valid class type in the validation check.
Remove NSNull as a valid class type in the validation check.

AzureAD/azure-activedirectory-library-for-objc,msftberlin/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,msintuneappsdk/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,msintuneappsdk/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,msftberlin/azure-activedirectory-library-for-objc