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.md

@@ -338,7 +338,7 @@ CHACHA20_POLY1305 (per RFC 8439) supports plaintext up to 2^38 - 64 bytes.
 Implementations MAY impose stricter limits based on platform constraints
 (e.g., limiting to 2^32 - 1 bytes on 32-bit architectures).
 
-Implementations MUST reject empty payloads.
+Implementations MUST reject empty payloads (i.e. zero content items).
 
 #### Ciphertext
 

bip-0138/test_vectors/payload.json

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