Error msg about DataFrame concatenation

[harisang]

For the past few weeks, we have observed this error showing up in the logs.

ERROR (pod: dune-sync-order-rewards): /app/src/fetch/orderbook.py:91: FutureWarning: The behavior of DataFrame concatenation with empty or all-NA entries is deprecated. In a future version, this will no longer exclude empty or all-NA columns when determining the result dtypes. To retain the old behavior, exclude the relevant entries before the concat operation.

Although it doesn't seem to affect anything for now, we should address this soon.