Corrected a flaw in input-checking logic of toLowerCase and toUpperCase.

This PR is unrelated to any bug tickets, but is just something I noticed.
```js
inString = null
// null
typeof(inString) === "null"
// false
inString === null
// true
```

Enyo-DCO-1.1-Signed-off-by: Blake Stephens <blake.stephens@lge.com>