fix(node): do not discard InitObservation when some keys don't match

[ffakenz]

Fixes #529

⛵ Make InitTx observable, by being less strict on the discarding rule.

⛵ Now we only discard in case the party tokens are not found in the minted assets for the InitTx.

⛵This should make it easier to see if a Head has been initialized or if there is a misconfiguration.