qt: Fix regression in TransactionTableModel

Since #17993 a crash is possible on exit.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

src/qt/transactionrecord.cpp

@@ -234,6 +234,7 @@ void TransactionRecord::updateStatus(const interfaces::WalletTxStatus& wtx, cons
 
 bool TransactionRecord::statusUpdateNeeded(const uint256& block_hash) const
 {
+    assert(!block_hash.IsNull());
     return status.m_cur_block_hash != block_hash || status.needsUpdate;
 }