To avoid the warning "Undefined array key "iss_allowed_domains" after converting
requested to merge issue/openid_connect-3375886:3375886-add-update-hook-to-fix-missing-iss-allowed-domains into 3.x
To avoid the warning "Undefined array key "iss_allowed_domains" after converting client configuration objects to configuration entities, we add an update hook for adding the missing "iss_allowed_domains" configuration if it is empty.
Closes #3375886