[TS+Java] Ensure we don't try to add a reference from `null`.

[TS+Java] Ensure we don't try to add a reference from `null`.

At least in `Event::PaymentPathFailed` if we try to map an event
with no `retry` we'll hit a `NullPointerException` as we'll try to
add a reference from the `null` retry back to the `Event` itself.

The simple fix is to simply exhaustively check for `null` before
adding references everywhere.