bip138: clarify rejection of empty payloads

Clarify that an empty payload contains zero content items. Add a
vector that rejects a payload consisting only of the 0x00 terminator.

bip-0138/test_vectors/payload.json

@@ -9,5 +9,10 @@
         "content": "61"
       }
     ]
+  },
+  {
+    "description": "Payload with no content item.",
+    "valid": false,
+    "payload": "00"
   }
 ]