查询结果为空flexdashboard时显示的消息

Question

我有一个带有反应表的flexdashboard。用户可以选择日期并期望结果，但是，如果我的查询为空，如何解释用户没有可用数据？如何退出该代码？

---
title: "Dashboard"
output: 
  flexdashboard::flex_dashboard:
    orientation: rows
    vertical_layout: fill
runtime: shiny
---

```{r setup, include=FALSE}
library(flexdashboard)
library(DBI)
library(RPostgreSQL)
library(tidyverse)
```

```{r}
dataset <- eventReactive(input$button_execute, {
  date_start <- input$input_date_start
  date_end <- input$input_date_end
  drv <- dbDriver("PostgreSQL")
  query <- "SELECT * FROM mytable"
  conection <- dbConnect(drv, 
                       dbname = "xxx", 
                       host = "xxx",
                       user = "xxx",
                       password = "xxx",
                       port = 9999)
  rs <- dbSendQuery(conection, query)
  fetched <- dbFetch(rs)
  fetched <- fetched %>% mutate(colnew = sum(colmoney)) 
  dbDisconnect(conection)
})
```

我尝试了以下操作：

if(nrow(fetched) != 0) then {
   # execute the mutate 
} else { 
   # say no data
}

我的问题是：有没有一种方法可以在flexdashboard中退出并避免呈现反应式表？