## isalpha
### Instructions
Write a function that returns `true` if the `string` passed in parameter only contains alphanumerical characters or is empty, and returns `false` otherwise.
### Expected function
```go
func IsAlpha(s string) bool {
}
```
### Usage
Here is a possible program to test your function :
```go
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
false
true
$
```