bolt12: Add test vector for missing offer_amount with offer_currency

Adds a test case to verify that offers with offer_currency set but
offer_amount missing are correctly rejected, as required by BOLT 12
specification section "Requirements For Offers":

"if `offer_currency` is set and `offer_amount` is not set:
 - MUST NOT respond to the offer."

Author: erickcestari

bolt12/offers-test.json

@@ -573,6 +573,11 @@
     "valid": false,
     "bolt12": "lno1pqpzwyqkyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
   },
+  {
+    "description": "Missing offer_amount with offer_currency",
+    "valid": false,
+    "bolt12": "lno1qcp4256ypgx9getnwss8vetrw3hhyuckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
   {
     "description": "Missing offer_issuer_id and no offer_path",
     "valid": false,