rpc: return empty txid get_outs rather than 00..00 when not requested

It's more obvious there's no txid, and it saves space

src/rpc/core_rpc_server.cpp

@@ -771,6 +771,7 @@ namespace cryptonote
       outkey.mask = epee::string_tools::pod_to_hex(i.mask);
       outkey.unlocked = i.unlocked;
       outkey.height = i.height;
+      if (req.get_txid)
         outkey.txid = epee::string_tools::pod_to_hex(i.txid);
     }