OnFail
Brokenness policy for a failed conversion. Absence is ALWAYS type-driven (nullable/default) — never a policy.
Fallback ladder, in order: converted > default > null/not-a-member > error. A sanctioned null — an OrNull converter variant declaring "this input has no legitimate counterpart" (see MapTypeConverter) — is legitimate flow, not a failure, so it is exempt even under Throw. Skip applies to collection elements only.