Golang template variable isset

I have created a function to check if a variable is defined:

fm["isset"] = func(a interface{}) bool {
        if a == nil || a == "" || a == 0 {
            fmt.Println("is not set")
            return false
        }
        fmt.Println("is set")
        return false
    }

tmpl :=  template.Must(template.New("").Funcs(fm).ParseFiles("templates/header.html"))

err := tmpl.ExecuteTemplate(w, "header", templateData)

In the template I have:

    email is set

This function works if the variable is contained by the templateData (which is a custom struct that contains a map and a string), but it gives me an error if the variable doesn't exist.

The error is:

executing "header" at <.Email>: can't evaluate field Email in type base.customData

In my case "base.go" is the handler and "customData" is defined by: type customData struct{..}.

I want to be able to reuse templates and to display some sections only if some variables are sent from the handler. Any idea how can I implement a variable isset check on the template side?

I also tried using: do stuff but this also gives me the same error.

Any idea?