ethclient: serialize negative block number as "pending" (#21177)

Fixes #21175

Co-authored-by: sammy007 <sammy007@users.noreply.github.com>
Co-authored-by: Adam Schmideg <adamschmideg@users.noreply.github.com>

ethclient/ethclient.go

@@ -282,6 +282,10 @@ func toBlockNumArg(number *big.Int) string {
 	if number == nil {
 		return "latest"
 	}
+	pending := big.NewInt(-1)
+	if number.Cmp(pending) == 0 {
+		return "pending"
+	}
 	return hexutil.EncodeBig(number)
 }