eth/fetcher: fix flaky test by improving event unsubscription (#33950)

Eth currently has a flaky test, related to the tx fetcher.

The issue seems to happen when Unsubscribe is called while sub is nil.
It seems that chain.Stop() may be invoked before the loop starts in some
tests, but the exact cause is still under investigation through repeated
runs. I think this change will at least prevent the error.

eth/fetcher/tx_fetcher.go

@@ -442,7 +442,9 @@ func (f *TxFetcher) loop() {
 	if f.chain != nil {
 		headEventCh = make(chan core.ChainEvent, 10)
 		sub := f.chain.SubscribeChainEvent(headEventCh)
-		defer sub.Unsubscribe()
+		if sub != nil {
+			defer sub.Unsubscribe()
+		}
 	}
 
 	for {