callback is nil not trace

Author: lbbniu

tars/tools/tars2go/gen_go.go

@@ -1257,6 +1257,9 @@ func (gen *GenGo) genIFProxyFun(interfName string, fun *FunInfo, withContext boo
 
 	// trace
 	if !isOneWay && !withoutTrace {
+		if isAsync {
+			c.WriteString(`if callback != nil {`)
+		}
 		c.WriteString(`
 trace, ok := current.GetTarsTrace(tarsCtx)
 if ok && trace.Call() {
@@ -1280,6 +1283,9 @@ if ok && trace.Call() {
 	}
 	tars.Trace(trace.GetTraceKey(tarstrace.EstCS), tarstrace.AnnotationCS, tars.GetClientConfig().ModuleName, obj.servant.Name(), "` + fun.Name + `", 0, traceParam, "")
 }`)
+		if isAsync {
+			c.WriteString(`}`)
+		}
 		c.WriteString("\n\n")
 	}
 	c.WriteString(`var statusMap map[string]string