Fix potential syntax err (#37)

* Fix potential syntax err

Some generated code may lead to
proto/server.gen.go:568:6: no new variables on left side of :=

* Basic example: Add a method with some request args but no return args

* Fix the syntax error

_examples/golang-basics/example.ridl

@@ -76,3 +76,4 @@ service ExampleService
   - Version() => (version: Version)
   - GetUser(header: map<string,string>, userID: uint64) => (user: User)
   - FindUser(s: SearchFilter) => (name: string, user: User)
+  - LogEvent(event: string)

_examples/golang-basics/main.go

@@ -90,3 +90,7 @@ func (rpc *ExampleServiceRPC) FindUser(ctx context.Context, s *SearchFilter) (st
 		Username: name,
 	}, nil
 }
+
+func (rpc *ExampleServiceRPC) LogEvent(ctx context.Context, event string) error {
+	return nil
+}

server.go.tmpl

@@ -74,7 +74,7 @@ func (s *{{$serviceName}}) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 {{range .Methods }}
 func (s *{{$serviceName}}) serve{{ .Name | firstLetterToUpper }}JSON(ctx context.Context, w http.ResponseWriter, r *http.Request) {
-	{{ if .Inputs|len}}
+	{{ if gt (len .Inputs) 0 -}}
 	reqBody, err := io.ReadAll(r.Body)
 	if err != nil {
 		RespondWithError(w, ErrorWithCause(ErrWebrpcBadRequest, fmt.Errorf("failed to read request data: %w", err)))
@@ -96,13 +96,13 @@ func (s *{{$serviceName}}) serve{{ .Name | firstLetterToUpper }}JSON(ctx context
 	ctx = context.WithValue(ctx, MethodNameCtxKey, "{{.Name}}")
 
 	// Call service method implementation.
-	{{range $i, $output := .Outputs}}ret{{$i}}, {{end}}err := s.{{$name}}.{{.Name}}(ctx{{range $i, $_ := .Inputs}}, reqPayload.Arg{{$i}}{{end}})
+	{{range $i, $output := .Outputs}}ret{{$i}}, {{end}}err {{if or (eq (len .Inputs) 0) (gt (len .Outputs) 0)}}:{{end}}= s.{{$name}}.{{.Name}}(ctx{{range $i, $_ := .Inputs}}, reqPayload.Arg{{$i}}{{end}})
 	if err != nil {
 		RespondWithError(w, err)
 		return
 	}
 
-	{{- if .Outputs | len}}
+	{{- if gt (len .Outputs) 0}}
 
 	respPayload := struct {
 	{{- range $i, $output := .Outputs}}