## isalpha
### Instructions
Write a function that returns `true` if the `string` passed as the parameter only contains alphanumerical characters or is empty, otherwise, and returns `false`.
### Expected function
```go
func IsAlpha(s string) bool {
}
```
### Usage
Here is a possible program to test your function :
package main
import (
"fmt"
"piscine"
)
func main() {
fmt.Println(piscine.IsAlpha("Hello! How are you?"))
fmt.Println(piscine.IsAlpha("HelloHowareyou"))
fmt.Println(piscine.IsAlpha("What's this 4?"))
fmt.Println(piscine.IsAlpha("Whatsthis4"))
And its output :
```console
$ go run .
false
true
$