bip138: clarify receive-only descriptor sets

Make change_descriptor explicitly optional for non-multipath descriptors.
Add a receive-only document vector with no change descriptor or optional
metadata; the existing vectors cover a receive/change pair and multipath.

bip-0138/test_vectors/bip380_descriptor_backup.json

@@ -29,5 +29,17 @@
         }
       ]
     }
+  },
+  {
+    "description": "Receive-only descriptor set without optional metadata",
+    "valid": true,
+    "document": {
+      "version": 1,
+      "descriptor_sets": [
+        {
+          "descriptor": "wpkh([d34db33f/84h/1h/0h]tpubDC5FSnBiZDMmhiuCmWAYsLwgLYrrT9rAqvTySfuCCrgsWz8wxMXUS9Tb9iVMvcRbvFcAHGkMD5Kx8koh4GquNGNTfohfk7pgjhaPCdXpoba/0/*)"
+        }
+      ]
+    }
   }
 ]