Get payment method ID from offerPayload.

Was getting an NPE with closed trades older than v1.7.0.
makerPaymentMethodId was new at that point (null for older trades).
makerPaymentAccountPayload can be null for trades where we have
removed account info for privacy reasons (see #6001).
The payment method ID can always be obtained from offerPayload.

core/src/main/java/bisq/core/trade/model/bisq_v1/Contract.java

@@ -298,8 +298,8 @@ public byte[] getHashOfPeersPaymentAccountPayload(PubKeyRing myPubKeyRing) {
     }
 
     public String getPaymentMethodId() {
-        // Either makerPaymentMethodId is set or available in makerPaymentAccountPayload
-        return makerPaymentMethodId != null ? makerPaymentMethodId : Objects.requireNonNull(makerPaymentAccountPayload).getPaymentMethodId();
+        // Either makerPaymentMethodId is set or available in offerPayload
+        return makerPaymentMethodId != null ? makerPaymentMethodId : Objects.requireNonNull(getOfferPayload()).getPaymentMethodId();
     }
 
     public Coin getTradeAmount() {