ethclient: omit empty address filter field in toFilterArg

Previously, an empty (non-nil) []common.Address{} slice caused the
"address" field to be serialized as an empty JSON array [] in the
RPC request. Some Ethereum-compatible nodes (e.g. Hedera) reject this
empty array with an error.

According to the JSON-RPC specification the "address" field is
optional. Align with the spec by only including the field when at
least one address is present, treating []Address{} the same as nil.

Add a test case "with empty addresses slice" to TestToFilterArg
to prevent regression.

ethclient/ethclient.go

@@ -498,7 +498,11 @@ func (ec *Client) SubscribeFilterLogs(ctx context.Context, q ethereum.FilterQuer
 
 func toFilterArg(q ethereum.FilterQuery) (interface{}, error) {
 	arg := map[string]interface{}{}
-	if q.Addresses != nil {
+	// Only include "address" when there are actual address filters.
+	// An empty slice is treated the same as nil (no filter), and omitting
+	// the field avoids sending "address":[] to nodes that reject empty arrays
+	// (e.g. Hedera, some non-Geth implementations).
+	if len(q.Addresses) > 0 {
 		arg["address"] = q.Addresses
 	}
 	if q.Topics != nil {

ethclient/types_test.go

@@ -53,6 +53,22 @@ func TestToFilterArg(t *testing.T) {
 			},
 			nil,
 		},
+		{
+			// empty Addresses slice must be treated same as nil:
+			// the "address" field must be omitted so that non-Geth nodes
+			// (e.g. Hedera) do not reject the request with an error.
+			"with empty addresses slice",
+			ethereum.FilterQuery{
+				Addresses: []common.Address{},
+				FromBlock: big.NewInt(1),
+				ToBlock:   big.NewInt(2),
+			},
+			map[string]interface{}{
+				"fromBlock": "0x1",
+				"toBlock":   "0x2",
+			},
+			nil,
+		},
 		{
 			"without BlockHash",
 			ethereum.FilterQuery{