ethclient: fix tx sender cache miss detection (#23877)

This fixes a bug in TransactionSender where it would return the
zero address for transactions where the sender address wasn't
cached already.

Co-authored-by: Felix Lange <fjl@twurst.com>

ethclient/ethclient.go

@@ -233,6 +233,8 @@ func (ec *Client) TransactionSender(ctx context.Context, tx *types.Transaction,
 	if err == nil {
 		return sender, nil
 	}
+
+	// It was not found in cache, ask the server.
 	var meta struct {
 		Hash common.Hash
 		From common.Address