accounts/abi/bind: add void if no return args specified (ethereum#21002)

* accounts/abi/bind: add void if no return args specified

Currently the java generator generates invalid input on pure/view functions
that have no return type. e.g. `function f(uint u) view public {}`
This is not a problem in practice as people rarely ever write functions like this.

* accounts/abi/bind: use elseif instead of nested if

Author: MariusVanDerWijden

accounts/abi/bind/template.go

@@ -624,7 +624,7 @@ import java.util.*;
 	// {{.Normalized.Name}} is a free data retrieval call binding the contract method 0x{{printf "%x" .Original.ID}}.
 	//
 	// Solidity: {{.Original.String}}
-	public {{if gt (len .Normalized.Outputs) 1}}{{capitalise .Normalized.Name}}Results{{else}}{{range .Normalized.Outputs}}{{bindtype .Type $structs}}{{end}}{{end}} {{.Normalized.Name}}(CallOpts opts{{range .Normalized.Inputs}}, {{bindtype .Type $structs}} {{.Name}}{{end}}) throws Exception {
+	public {{if gt (len .Normalized.Outputs) 1}}{{capitalise .Normalized.Name}}Results{{else if eq (len .Normalized.Outputs) 0}}void{{else}}{{range .Normalized.Outputs}}{{bindtype .Type $structs}}{{end}}{{end}} {{.Normalized.Name}}(CallOpts opts{{range .Normalized.Inputs}}, {{bindtype .Type $structs}} {{.Name}}{{end}}) throws Exception {
 		Interfaces args = Geth.newInterfaces({{(len .Normalized.Inputs)}});
 		{{range $index, $item := .Normalized.Inputs}}Interface arg{{$index}} = Geth.newInterface();arg{{$index}}.set{{namedtype (bindtype .Type $structs) .Type}}({{.Name}});args.set({{$index}},arg{{$index}});
 		{{end}}