Capitalize first letter: safe against empty string

`""[0]` -> `undefined`
`"".slice(0, 1)` -> `""`

snippets/capitalize-first-letter.md

@@ -3,5 +3,5 @@
 Use `toUpperCase()` to capitalize first letter, `slice(1)` to get the rest of the string.
 
 ```js
-var capitalize = str => str[0].toUpperCase() + str.slice(1);
+var capitalize = str => str.slice(0, 1).toUpperCase() + str.slice(1);
 ```