Detect and don't try to parse empty txes

If we pass the blob into parse_and_validate_tx_from_blob we get
`deserialization or varint failed` error logs, which are harmless bug
annoying.  This catches the empty tx blob case earlier and logs at a
lesser severity.

src/cryptonote_core/cryptonote_core.cpp

@@ -1120,6 +1120,12 @@ namespace cryptonote
       tx_info.tvc.m_too_big = true;
       return;
     }
+    else if (tx_info.blob->empty())
+    {
+      LOG_PRINT_L1("WRONG TRANSACTION BLOB, blob is empty, rejected");
+      tx_info.tvc.m_verifivation_failed = true;
+      return;
+    }
 
     tx_info.parsed = parse_and_validate_tx_from_blob(*tx_info.blob, tx_info.tx, tx_info.tx_hash);
     if(!tx_info.parsed)