## isalpha
### Instructions
Write a function that returns `true` if the `string` passed in parameter only contains alphanumerical characters or is empty, and that 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
student@ubuntu:~/[[ROOT]]/test$ go build
student@ubuntu:~/[[ROOT]]/test$ ./test
false
true
false
true
student@ubuntu:~/[[ROOT]]/test$
```
