Empty numericInput gets converted to NA

This fixes the bug from: https://groups.google.com/forum/?fromgroups=#!topic/shiny-discuss/41-LYu3aw-0

Sample test code:

```r
# shinyRun function defined at:
# https://gist.github.com/wch/1bab36af24754a896293
source_gist('1bab36af24754a896293')

shinyRun(
#--------------------------------------------------------------
`server.R` = quote({
  shinyServer(function(input, output) {
    output$numtext <- renderText({
      input$num
    })
  })
}),
#--------------------------------------------------------------
`ui.R` = quote({
  library(shinyIncubator)

  shinyUI(pageWithSidebar(
    headerPanel("Type a number"),
    sidebarPanel(
      numericInput("num", "Number:", 0)
    ),
    
    # Show a plot of the generated distribution
    mainPanel(
      textOutput('numtext')
    )
  ))
})
)
```