isogram: "invert if" case not caught

[tehsphinx]

package isogram

import "unicode"

// IsIsogram takes a string as input and decides whether it's an isogram or not
func IsIsogram(s string) bool {
	m := make(map[rune]int, len(s))
	for _, r := range s {
		if unicode.Is(unicode.Letter, r) {
			if _, ok := m[unicode.ToLower(r)]; ok {
				return false
			}
			m[unicode.ToLower(r)]++
		}
	}
	return true
}

This should show a comment about inverting the if for happy path on the left.