itest: fix flake in testRevokedCloseRetributionZeroValueRemoteOutput

We need to mine an empty block as the tx may already have entered the
mempool. This should be fixed once we start using the sweeper to handle
the justice tx.

itest/lnd_revocation_test.go

@@ -612,7 +612,7 @@ func revokedCloseRetributionRemoteHodlCase(ht *lntest.HarnessTest,
 		// transactions will be in the mempool at this point, we pass 0
 		// as the last argument, indicating we don't care what's in the
 		// mempool.
-		ht.MineBlocks(1)
+		ht.MineEmptyBlocks(1)
 		err = wait.NoError(func() error {
 			txid, err := findJusticeTx()
 			if err != nil {