[fix] Support PoL Withdraw Events
The withdraw events emitted by protocol owned liquidity do not correspond to a transaction and have no memo. This allows null to avoid fatal on the batch write in these cases:
FTL 2022-10-04 18:18:25 Inserter.Flush() failed. Marking BatchInserter as failed and exiting to switch to TxInserter. error="ERROR: null value in column \"memo\" of relation \"_hyper_190_6071_chunk\" violates not-null constraint (SQLSTATE 23502)"