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_test.go

@@ -97,6 +97,22 @@ func TestToFilterArg(t *testing.T) {
 			},
 			nil,
 		},
+		{
+			"with negative fromBlock and negative toBlock",
+			ethereum.FilterQuery{
+				Addresses: addresses,
+				FromBlock: big.NewInt(-1),
+				ToBlock:   big.NewInt(-1),
+				Topics:    [][]common.Hash{},
+			},
+			map[string]interface{}{
+				"address":   addresses,
+				"fromBlock": "pending",
+				"toBlock":   "pending",
+				"topics":    [][]common.Hash{},
+			},
+			nil,
+		},
 		{
 			"with blockhash",
 			ethereum.FilterQuery{